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Post-exertional malaise is often described as a crash after activity. But what happens when you haven't recovered from one crash before the next demand arrives? People with ME/CFS and Long COVID increasingly use the term “rolling PEM” to describe a pattern in which episodes of post-exertional worsening overlap until it becomes difficult to tell where one ends and another begins. Sometimes post-exertional malaise has an obvious beginning. You attend an appointment. The next day, you crash. You reduce your activity. Several days later, you begin returning toward your usual baseline. The connection is relatively easy to see: Activity → delayed worsening → recovery. But chronic illness doesn't always happen in such neat episodes. What if you have to shower while you're still recovering? What if you have another medical appointment two days later? What if you're working, parenting, studying, cooking, talking, thinking, managing symptoms, and simply trying to get through everyday life? What happens when the next demand arrives before your body has recovered from the previous one? People in ME/CFS and Long COVID communities sometimes describe this experience as: rolling PEM. And understanding the concept may help explain why some crashes seem to last indefinitely—or why someone's baseline can become increasingly difficult to identify. First: What Is PEM? PEM stands for post-exertional malaise, also called post-exertional symptom exacerbation. It is a hallmark feature of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and is also reported by some people with Long COVID. PEM is not simply feeling tired after exercise. It is an abnormal worsening of symptoms following exertion that would previously have been tolerated. The triggering activity doesn't have to be strenuous. It may be: physical cognitive emotional social And one of PEM's defining characteristics is that the worsening may be delayed. Someone might complete an activity and feel reasonably okay afterward. Hours later—or the following day—their symptoms intensify. The deterioration can involve far more than fatigue. Brain fog may increase. Pain may worsen. Sleep may become less restorative. Orthostatic symptoms may intensify. Someone may develop headaches, weakness, sensory sensitivity, flu-like symptoms, or a general worsening of their existing illness. Recovery may take days, weeks, or longer. The Simple Version of PEM Imagine that your usual functional level looks like this: Baseline → Activity → PEM → Recovery → Baseline You do more than your body can currently tolerate. Your symptoms worsen. You reduce activity and recover. Eventually, you return toward your previous level. That model is useful. But it assumes something important: You get enough recovery time before the next significant exertion occurs. Real life often doesn't allow that. What Is “Rolling PEM”? Rolling PEM is an informal but increasingly recognized term used to describe a situation in which additional exertion occurs before someone has adequately recovered from previous post-exertional worsening. Instead of: Activity → PEM → Recovery the pattern may look more like: Activity → PEM → More activity → More PEM → Necessary activity → More PEM → Partial recovery → More activity → More PEM Eventually, the individual episodes become difficult to distinguish. There isn't an obvious crash anymore. There is just: “I feel terrible all the time.” That's what makes rolling PEM so difficult to recognize. The Term Is Beginning to Appear Beyond Patient Communities “Rolling PEM” has largely been community language rather than a formal medical diagnosis. But the concept is beginning to appear in professional literature. A 2026 expert statement on home-based care for severe ME/CFS described a pattern in which recovery is repeatedly interrupted by new exertion-induced deterioration, referring to this as “rolling PEM.” That doesn't make rolling PEM a separate disease or diagnostic category. Instead, the phrase provides useful shorthand for a phenomenon clinicians and patients need to understand: recovery can potentially be interrupted by additional exertion. Why Delayed PEM Makes This Especially Difficult Imagine this sequence. Monday You attend a medical appointment. You feel tired afterward but not dramatically worse. Tuesday You answer emails, take a shower, and prepare food. You feel unusually exhausted and assume Tuesday's activities caused it. But Monday's delayed PEM may already be beginning. Wednesday Your symptoms worsen substantially. You still need to eat, use the bathroom, communicate with family, manage medications, and attend to basic responsibilities. Those activities occur while your system is already struggling to recover. Thursday You're worse again. Now what caused the crash? Monday's appointment? Tuesday's shower? Wednesday's basic activities? All of them? At that point, identifying a single trigger becomes nearly impossible. This is one reason rolling PEM can feel as though it appeared “out of nowhere.” The problem may not be one dramatic exertion. It may be accumulated activity combined with incomplete recovery. Exertion Doesn't Mean Exercise This distinction becomes even more important when discussing rolling PEM. People often hear exertion and imagine exercise. Running. Cycling. Weightlifting. Walking long distances. But PEM can follow much smaller demands. Cognitive activity counts. Social activity counts. Emotional demands count. And everyday physical tasks count. That means an apparently restful day might still contain substantial physiological demand. You might spend the day in bed while: answering messages, researching symptoms, having an emotionally difficult conversation, participating in a telehealth appointment, making decisions, filling out disability paperwork, scrolling through highly stimulating information, or concentrating on a television program. Your step count might say: 312 steps. That doesn't necessarily mean the day contained only 312 steps' worth of demand. The Invisible Activity Problem This is one of the biggest challenges in understanding PEM. We are accustomed to measuring activity through movement. But a person's total daily load can include: Physical loadWalking, standing, showering, dressing, preparing food. Cognitive loadReading, writing, studying, working, planning, decision-making. Orthostatic loadSitting upright, standing, traveling, waiting in lines. Social loadConversation, appointments, visitors, gatherings. Emotional loadStress, conflict, anxiety-provoking situations. Sensory loadNoise, bright lights, busy environments, screens. Looking only at exercise can therefore dramatically underestimate someone's total exertion. Necessary Activities Still Require Energy One of the hardest realities of severe chronic illness is that not all exertion is optional. You still have to eat. You still have to use the bathroom. You may need to shower. You may need medical care. You may have children. You may have to work. You may need to communicate with insurance companies, pharmacies, doctors, schools, employers, or disability systems. Sometimes the activity that exceeds someone's current capacity isn't recreational. It's simply being alive. This is especially important when discussing severe or very severe ME/CFS. Telling someone to “just rest” can overlook the fact that even basic activities of daily living may represent meaningful exertion when someone's available capacity is extremely limited. Why Baseline Can Become Hard to Identify People with chronic illness frequently talk about their baseline. Baseline usually means something like: the level of symptoms and function I experience when I'm relatively stable. But what if you haven't been stable for weeks? That's when people begin saying things like: “I don't know what my baseline is anymore.” If episodes of symptom worsening continually overlap, today's symptoms may contain the effects of multiple previous exertions. That can make it difficult to answer questions such as: “How much activity can I tolerate?” “What triggered this crash?” “Am I getting better?” “Is this my new baseline?” “Am I still in PEM?” There may not be an immediate answer. A Flare, PEM, and a Relapse Aren't Necessarily the Same Thing These words are often used interchangeably online, but clinical guidance makes some useful distinctions. NICE describes a flare-up as a temporary worsening of symptoms beyond ordinary day-to-day variation. PEM frequently contributes to flare-ups. A relapse, however, is a more sustained and significant deterioration requiring a substantial adjustment to someone's energy management. Importantly, NICE notes that a relapse can potentially lead to a longer-term reduction in a person's energy limits. And early in a deterioration, it may not be obvious whether someone is experiencing a temporary flare or something more prolonged. That's another reason repeatedly overlapping crashes deserve attention. Why “I Barely Did Anything” Can Be Misleading Imagine someone spends an afternoon at a family gathering. They sit most of the time. From the outside, they didn't do much. But physiologically, the afternoon might have included: getting dressed, showering, walking to the car, traveling, sitting upright, maintaining conversation, processing noise, following multiple conversations, eating, regulating temperature, walking back to the car, traveling home, and getting ready for bed. The activity wasn't one thing. It was a stack of demands. And if the person was already recovering from previous exertion, the available margin may have been much smaller than usual. The Activity Stack One useful way to think about rolling PEM is through an activity stack. Instead of asking: “What big thing did I do?” ask: “What has my body been asked to do over the last several days?” For example: Monday: doctor's appointment Tuesday: shower + phone calls Wednesday: poor sleep + grocery delivery + cooking Thursday: family visit Friday: paperwork + prolonged screen time None of those activities necessarily looks enormous individually. But the body doesn't reset to zero at midnight. If recovery from Monday is still occurring on Tuesday, and Tuesday adds another demand, and Wednesday adds another, the total physiological picture may be very different from looking at each day independently. This Is Not About Becoming Afraid of Activity Understanding rolling PEM should not turn every movement, conversation, or task into something frightening. The goal isn't to teach people that activity is dangerous. It is to recognize that capacity can be limited and recovery matters. For people experiencing PEM, major clinical guidance recommends individualized energy management rather than automatically increasing activity according to a predetermined schedule. That process is often referred to as pacing. What Pacing Actually Means Pacing is sometimes misunderstood as: “Do less.” A better description is: managing activity in relation to available capacity. NICE describes energy management as incorporating all types of activity—including physical, cognitive, emotional, and social activity. The goal is to reduce the risk of worsening symptoms by repeatedly exceeding the individual's current energy limit. That means pacing isn't necessarily about staying in bed. It can involve changing: the duration of an activity, the intensity, the position in which it is performed, how activities are spaced, how much recovery occurs afterward, or how many demanding activities happen on the same day. Rest Before Exhaustion Can Feel Counterintuitive Most of us learn to rest after becoming tired. Work until you're tired. Exercise until you're tired. Clean until you're tired. Then recover. PEM complicates that strategy because the body's warning signal may arrive late. If symptoms are delayed by 12, 24, or even 48 hours, waiting until you feel terrible may mean the relevant limit was crossed much earlier. That's one reason activity and symptom tracking can sometimes be helpful. The goal isn't to obsess over every movement. It's to look for patterns that aren't obvious in real time. Tracking the Previous 48 Hours When symptoms suddenly worsen, instead of asking only: “What did I do today?” it may be useful to ask: “What happened yesterday?” and: “What happened the day before that?” Consider recording: physical activity upright time cognitive work social activity unusually stressful events sleep symptom severity when symptom worsening began how long recovery took Over time, the relationship between exertion and delayed symptoms may become easier to see. A diary cannot diagnose PEM or ME/CFS. But it can help reveal patterns that human memory isn't particularly good at detecting. What About Good Days? Good days can create another trap. You wake up feeling noticeably better. Naturally, you want to use that capacity. You answer the emails you've ignored. Clean the kitchen. Take a shower. Make dinner. Call someone. Maybe you even think: “I'm finally getting better.” Then the crash arrives. This doesn't mean improvement isn't real. But feeling better today doesn't necessarily tell you how much activity your body can recover from tomorrow. For people with delayed post-exertional symptoms, sustainable capacity and momentary capacity may not always be identical. Recovery Is Becoming an Important Research Question Scientists studying PEM are increasingly interested not only in what happens during exertion, but what happens afterward. A 2026 perspective on PEM in ME/CFS and Long COVID proposes examining the body's post-exertional physiological trajectory across time. Instead of measuring someone only before and immediately after activity, researchers can investigate what happens: hours later, the following day, and throughout recovery. This may be particularly important for understanding PEM because the defining abnormality may not simply be the body's immediate response to exertion. The recovery process itself may be abnormal. That is still an emerging scientific framework rather than a settled mechanism. But it offers an interesting way to think about rolling PEM. If recovery is incomplete when another demand arrives, the next recovery trajectory begins from a system that hasn't returned to its previous state. Can Rolling PEM Permanently Lower Your Baseline? This question comes up frequently in chronic-illness communities. The most accurate answer is: We don't fully know. NICE recognizes that relapses can sometimes lead to longer-term reductions in a person's energy limits. But that does not mean every episode of PEM permanently worsens ME/CFS. Nor can anyone reliably predict from a single crash whether someone's baseline will change. The important distinction is between: temporary symptom worsening, a longer flare, and a sustained relapse. Those outcomes aren't interchangeable. Rolling PEM Isn't a Personal Failure There is another reason this concept matters. People experiencing repeated crashes can easily conclude: “I'm terrible at pacing.” But pacing can become extraordinarily difficult when: symptoms are delayed, capacity fluctuates, activities are unavoidable, and the threshold that triggers PEM isn't directly measurable. You cannot always know in real time that you've exceeded your current capacity. Sometimes the information arrives a day later. Understanding that limitation makes activity management less about perfect control and more about identifying patterns. When Symptoms Keep Getting Worse Not every prolonged deterioration should automatically be attributed to PEM. New or substantially worsening symptoms can have other causes. Infection, medication effects, sleep disorders, anemia, thyroid problems, nutritional deficiencies, cardiovascular issues, autoimmune disease, and many other conditions can produce fatigue or worsening function. A significant or unexplained change from someone's usual pattern deserves medical evaluation rather than automatically being labeled a crash. PEM can coexist with other medical problems. The Most Useful Question May Be About Recovery When trying to understand activity with ME/CFS or Long COVID, we often ask: “How much can I do?” Another question may be just as important: “How well can I recover from what I'm doing?” Because sustainable activity isn't simply activity you can complete. It's activity from which your body can adequately recover. That distinction may be one of the most important ideas in understanding rolling PEM. The Bottom Line “Rolling PEM” isn't a separate diagnosis. It is a useful term for a pattern in which new exertion may occur before recovery from previous post-exertional worsening is complete. Because PEM can be delayed, and because exertion includes cognitive, emotional, social, orthostatic, and physical demands, the individual triggers can become surprisingly difficult to identify. Instead of: activity → crash → recovery the experience can become: activity → crash → activity → crash → partial recovery → activity → crash until there is no obvious boundary between episodes. Understanding the pattern shifts attention toward something that is becoming increasingly important in PEM research: recovery. Sometimes the most important question isn't whether you can complete an activity today. It's whether your body can recover from it tomorrow. A Note From Complexa Wellness Complex chronic illness often makes ordinary ideas—activity, rest, fatigue, and recovery—far more complicated. At Complexa Wellness, our goal is to translate emerging research and confusing chronic-illness terminology into accessible, evidence-informed education without turning developing science into certainty. Wellness made simpler. This article is for educational purposes only and is not intended to diagnose, treat, cure, or prevent any disease. Significant, new, or unexplained changes in symptoms or function should be discussed with an appropriate healthcare professional. References Centers for Disease Control and Prevention. Strategies to Prevent Worsening of Symptoms: ME/CFS. Centers for Disease Control and Prevention. Symptoms of ME/CFS. National Institute for Health and Care Excellence (NICE). Myalgic encephalomyelitis (or encephalopathy)/chronic fatigue syndrome: diagnosis and management. NICE Guideline NG206. A cardiometabolic perspective on post-exertional malaise in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and Long COVID. Cardiovascular Diabetology. Published August 20, 2026. Guidance on Home-Based Care for People with Severe ME/CFS: A Transdisciplinary Expert Statement. Wiener Medizinische Wochenschrift. 2026.

Fatigue after standing or activity is common with POTS. Post-exertional malaise (PEM) is something different. When POTS, ME/CFS, and Long COVID overlap, however, telling them apart can become surprisingly difficult. You go to the grocery store. While you're there, your heart rate climbs. Your legs feel heavy. Thinking becomes harder. By the time you get home, you're exhausted. You lie down, hydrate, and rest. Eventually, you begin to feel somewhat better. Another day, you do something that doesn't seem particularly strenuous. Maybe you take a shower, attend an appointment, work on your computer, or spend an afternoon with family. You feel reasonably okay afterward. Then, later that evening—or the next day—you feel dramatically worse. Your fatigue intensifies. Your brain fog becomes overwhelming. Your body aches. You may feel flu-like, weak, dizzy, or unusually sensitive to light and sound. Your normal symptoms become significantly worse, and recovery takes days. Both experiences might be described as: “Activity wiped me out.” But physiologically, they may not represent the same thing. Understanding the difference between POTS-related exertional or orthostatic fatigue and post-exertional malaise (PEM) can be particularly important for people living with POTS, ME/CFS, Long COVID, or combinations of these conditions. First: “Fatigue” Isn't One Symptom Fatigue is one of the most common words in chronic illness—and one of the least precise. When someone says, “I'm fatigued,” they might mean: sleepy physically weak mentally exhausted unable to remain upright heavy or leaden depleted after standing cognitively overwhelmed unable to sustain activity experiencing a whole-body symptom flare Those experiences can feel similar from the inside. But they may have very different triggers, timing, and recovery patterns. That's why the pattern surrounding the fatigue can sometimes tell us more than the word fatigue itself. What Does POTS Fatigue Feel Like? POTS—postural orthostatic tachycardia syndrome—is a disorder of the autonomic nervous system characterized by an excessive increase in heart rate when upright, together with symptoms of orthostatic intolerance. Fatigue is extremely common. Standing requires the body to compensate for gravity. When we become upright, blood shifts toward the lower body. Normally, the autonomic nervous system rapidly adjusts blood-vessel tone, heart function, and circulation to maintain adequate blood flow. In POTS, those adjustments don't function normally. Simply remaining upright can therefore represent a substantial physiological demand. That can produce symptoms such as: racing heart → dizziness → weakness → brain fog → heavy limbs → exhaustion Activities that combine standing, heat, and movement can be especially challenging. Think: showering, cooking, standing in line, shopping, cleaning, or walking through a large store. Someone may technically be capable of performing these activities while paying a considerable physiological price for doing them. Timing Can Provide an Important Clue One characteristic of orthostatic intolerance is that symptoms frequently appear during or relatively soon after the provoking activity. You stand. Symptoms increase. You continue standing. Symptoms get worse. You sit or lie down. Some symptoms may begin to improve. That doesn't mean POTS fatigue always disappears immediately after lying down. A demanding day can leave someone exhausted for much longer. But there is often a recognizable relationship between being upright and becoming symptomatic. PEM can behave very differently. What Is Post-Exertional Malaise? Post-exertional malaise is a worsening of symptoms following activity that would not have caused the same degree of illness before someone became sick. PEM is a defining feature of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and is also reported by a subset of people with Long COVID. Importantly, PEM isn't simply: “I exercised and got tired.” It can involve a broader worsening of the illness. Existing symptoms may intensify and new symptoms may appear. That can include increased fatigue, cognitive dysfunction, pain, sleep disturbance, flu-like feelings, orthostatic intolerance, sensory sensitivity, weakness, headaches, sore throat, or other symptoms. And one of PEM's most confusing characteristics is its timing. PEM Can Be Delayed Someone may perform an activity and initially think: “That went surprisingly well.” Hours later, they still feel okay. Then the deterioration begins. PEM can emerge hours after exertion and is commonly described as becoming particularly noticeable the following day. That delay makes it remarkably difficult to identify triggers. If you feel terrible Tuesday morning, the cause may not be what you did Tuesday morning. The relevant activity could have happened Monday afternoon. Or Monday morning. This is one reason people experiencing PEM sometimes repeatedly exceed their limits without realizing it. The body's consequences don't necessarily arrive at the same time as the activity. Physical Exercise Isn't the Only Possible Trigger The word exertion can also be misleading. PEM can follow physical activity, but physical exercise isn't the only form of exertion. Potential triggers may include: Physical exertionWalking, cleaning, exercising, shopping, or showering. Cognitive exertionWorking, studying, reading, completing paperwork, or solving complicated problems. Social exertionExtended conversations, gatherings, appointments, or events. Emotional exertionStressful situations or intense emotional experiences. Sensory loadBusy environments, bright lights, noise, or large amounts of stimulation may contribute to overall activity load for some people. A person can therefore experience a significant crash after a day in which their step count was extremely low. Activity isn't measured only in footsteps. POTS Fatigue and PEM Can Look Similar This is where things get complicated. Imagine two people who attend a medical appointment. Both spend an hour getting ready. Both travel to the clinic. Both sit in a waiting room. Both stand periodically. Both concentrate during a complicated conversation. Both travel home exhausted. The first person's symptoms become severe during the appointment. Their heart is racing, they are dizzy and their thinking becomes difficult. After getting home and lying down, symptoms gradually begin settling. That pattern could be consistent with orthostatic and autonomic stress associated with POTS. The second person also feels tired afterward. But the following morning, they wake substantially worse. Their cognition has deteriorated, their whole body feels ill, and multiple symptoms remain exacerbated for several days. That pattern raises the possibility of PEM. The activities were identical. The recovery patterns were not. A Useful Way to Think About the Difference Rather than asking only: “Did activity make me tired?” consider asking: When did symptoms appear? What symptoms worsened? How long did the worsening last? Did lying down substantially change the symptoms? Did I develop a broader illness flare later? The distinction often looks something like this: POTS-related orthostatic/exertional symptoms Symptoms often increase during upright activity or shortly afterward. Being upright, standing, heat, meals, or physical exertion may be prominent triggers. Tachycardia, dizziness, weakness, palpitations, brain fog, and heavy limbs may accompany the fatigue. Sitting or lying down may improve at least some symptoms. PEM Symptoms may worsen after a delay, although immediate worsening can also occur. The deterioration can involve multiple body systems rather than fatigue alone. Physical, cognitive, emotional, or social activity can contribute. Recovery may take days or longer. Resting or lying down may help manage symptoms but doesn't necessarily make the episode disappear quickly. These are patterns—not diagnostic rules. What If You Have Both? This is extremely important. POTS and ME/CFS can occur together. POTS and Long COVID can occur together. ME/CFS and Long COVID can overlap. Someone can therefore experience orthostatic intolerance during an activity and PEM afterward. For example: You stand in the kitchen preparing dinner. Your POTS symptoms worsen while standing. Your heart rate rises. Your legs become heavy. You feel dizzy. You sit down and improve somewhat. But the total physical and cognitive demand of preparing dinner exceeded your available capacity. The next day, you experience a broader and prolonged worsening of symptoms. The first response may have reflected orthostatic stress. The later response may resemble PEM. One doesn't exclude the other. Why This Distinction Matters for Exercise Advice This is where confusing fatigue with PEM can become particularly problematic. Physical conditioning can be an important part of management for some people with POTS, and carefully selected exercise or rehabilitation approaches may be recommended depending on the individual. ME/CFS is different. Major clinical guidance recognizes that fixed increases in activity that ignore a person's symptom-limited capacity can worsen symptoms in people experiencing PEM. That means advice as simple as: “You just need to exercise more.” can be inappropriate when PEM is present. At the same time, telling every person with POTS to avoid physical activity because exertion causes symptoms would also be an oversimplification. The correct approach depends on what condition or combination of conditions is actually driving the person's response to exertion. Deconditioning Makes the Conversation Even More Complicated People with chronic illness frequently hear the word deconditioning. Periods of inactivity can absolutely reduce cardiovascular fitness, muscle strength, and exercise tolerance. But deconditioning and PEM aren't interchangeable. A deconditioned person may become tired quickly during activity. They may experience shortness of breath, muscle soreness, and reduced endurance. PEM involves an abnormal worsening of symptoms following exertion that can be delayed and prolonged. A person can also be both deconditioned and experience PEM. These aren't mutually exclusive categories. That's why reducing every activity-related symptom to deconditioning can miss important information. The “Boom and Bust” Cycle Delayed symptoms can create another problem. Imagine you wake up feeling unusually good. You haven't felt this capable in days, so you try to catch up. You clean the kitchen. Answer emails. Run errands. Take a longer shower. Make dinner. Talk to a friend. You finally feel productive. Then the next day—or even later—you crash. After several days of recovery, you have another better day. So you catch up again. Another crash follows. This pattern is sometimes described as boom and bust. When consequences are delayed, today's available energy can be mistaken for evidence that today's activity level is sustainable. Those aren't necessarily the same thing. Tracking Patterns Can Be More Useful Than Tracking “Fatigue” Because these symptoms overlap, a simple symptom log can sometimes reveal patterns that memory misses. Instead of recording only: Fatigue: 8/10 it may be more informative to record: Activity: grocery shoppingUpright time: approximately 40 minutesSymptoms during activity: tachycardia, dizziness, leg heavinessSymptoms two hours later: tired but improvingSymptoms next morning: baseline Compare that with: Activity: two-hour family gatheringSymptoms during activity: mild fatigueSymptoms that evening: increasing brain fogSymptoms next morning: severe fatigue, pain, weakness, sensory sensitivityRecovery: three days Over time, the timing and shape of the response may become easier to recognize. A symptom diary cannot diagnose POTS, ME/CFS, Long COVID, or PEM. But it can provide much better information to discuss with a healthcare professional than the word “fatigue” alone. What About Heart Rate and Wearables? Some people use heart-rate monitors, smartwatches, step counters, or symptom-tracking apps to understand their activity patterns. These tools can sometimes provide useful context. For example, they may help someone recognize how much time they spend upright or how their heart rate responds to everyday tasks. But consumer wearables have important limitations. A smartwatch cannot currently diagnose PEM. A heart-rate threshold cannot definitively tell someone whether an activity will trigger a crash. And two activities with similar heart rates may place very different cognitive, emotional, sensory, or physiological demands on the body. Wearables are measurements. They aren't complete interpretations. One of the Most Important Clues May Be Recovery When trying to understand exertional symptoms, we often focus on the activity itself. Researchers studying PEM are increasingly interested in something else: What happens afterward? A healthy physiological system responds to exertion and then moves toward baseline. The trajectory of that recovery may be especially important in conditions involving exercise intolerance and PEM. Instead of asking only: “What happens while someone exercises?” researchers can ask: “What happens 4 hours later? 24 hours later? 48 hours later?” That shift may ultimately help scientists understand why two people can perform similar activities but experience radically different recoveries. So How Can You Tell the Difference? There isn't a perfect home test. And symptoms shouldn't be used to self-diagnose a complex chronic illness. But the timing, breadth, triggers, and duration of symptom worsening can provide useful clues. POTS-related orthostatic fatigue often has a strong relationship with being upright and may appear during the provoking activity. PEM often involves a broader deterioration following exertion, may be delayed, and can persist for substantially longer. And when POTS and ME/CFS or Long COVID overlap? Both patterns can happen to the same person. That's why the most useful question may not be: “Is this POTS or PEM?” It may be: “What does my body actually do during activity—and what happens in the hours and days afterward?” That is a much richer piece of information. The Bottom Line Feeling exhausted after activity doesn't automatically mean PEM. And experiencing POTS doesn't mean every episode of fatigue is caused by orthostatic intolerance. The pattern matters. POTS-related fatigue may intensify during standing or activity and often occurs alongside other orthostatic symptoms. PEM is characterized by a disproportionate worsening of symptoms following exertion, can be delayed, and may persist for days or longer. For people living with overlapping POTS, ME/CFS, or Long COVID, distinguishing those responses can be difficult—but understanding the difference may help make activity patterns, crashes, and recovery considerably less mysterious. Sometimes the most useful information isn't simply how tired you became. It's when you became tired, what else changed, and how long it took your body to recover. A Note From Complexa Wellness Complex chronic illness rarely follows simple rules. Symptoms can overlap across diagnoses, and words such as fatigue, exercise intolerance, crash, and PEM are often used to describe very different physiological experiences. At Complexa Wellness, our goal is to make those distinctions easier to understand through accessible, evidence-informed chronic-illness education. Wellness made simpler. This article is for educational purposes only and is not intended to diagnose, treat, cure, or prevent any disease. New, severe, or changing symptoms should be discussed with an appropriate healthcare professional. Suggested References Centers for Disease Control and Prevention. Strategies to Prevent Worsening of Symptoms: ME/CFS. National Institute for Health and Care Excellence (NICE). ME/CFS: Diagnosis and Management (NG206). Bateman L, Bested AC, Bonilla HF, et al. Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Essentials of Diagnosis and Management. Mayo Clinic Proceedings. 2021. Chung TH, Raj SR. Postural Orthostatic Tachycardia Syndrome (POTS): A Review. JAMA. 2026. Davenport TE, Stevens SR, VanNess MJ, Snell CR, Little T. Conceptual Model for Physical Therapist Management of Chronic Fatigue Syndrome/Myalgic Encephalomyelitis. Physical Therapy. 2010.

Brain fog is one of the most frustrating—and often least visible—symptoms reported by people with postural orthostatic tachycardia syndrome (POTS). New research is helping scientists investigate what may be happening in the brain. You know the information. You know the word you're trying to say. But suddenly you can't find it. You reread the same paragraph several times. Following a conversation becomes exhausting. Planning something that should be simple feels strangely complicated. After concentrating for a while, you feel as though you've done something physically strenuous—even though you've barely moved. People with POTS often call this brain fog. The phrase can sound deceptively mild. But the cognitive difficulties associated with POTS can interfere with school, work, conversations, reading, driving and everyday decision-making. And researchers are increasingly investigating whether some of those cognitive symptoms may be connected to changes in cerebral blood flow and autonomic regulation inside the brain. POTS Is About More Than Heart Rate POTS is a chronic disorder of the autonomic nervous system characterized by an excessive increase in heart rate when a person becomes upright, together with symptoms of orthostatic intolerance. But heart rate is only one part of the condition. People with POTS may experience symptoms involving multiple systems, including dizziness, palpitations, fatigue, exercise intolerance, gastrointestinal symptoms, temperature-regulation difficulties and cognitive dysfunction. A major review published in JAMA in August 2026 describes POTS as a chronic autonomic disorder with multisystem symptoms. That broader view is important. The autonomic nervous system helps regulate many processes that happen without conscious effort—including heart rate, blood pressure, digestion, temperature regulation and aspects of circulation. That includes maintaining an adequate supply of blood to the brain as conditions change. What Happens to Circulation When You Stand? Standing looks simple. Physiologically, it isn't. When you move from lying down to standing, gravity shifts blood toward the lower part of the body. Your cardiovascular and autonomic systems respond rapidly to help maintain circulation and blood pressure. Blood vessels constrict. Heart function adjusts. Multiple reflex systems coordinate to keep blood moving where it needs to go. In POTS, some of that regulation doesn't function normally. That is one reason researchers have become interested in a seemingly simple question: What is happening to blood flow in the brain? New Research Is Giving Scientists a Closer Look A 2026 study published in Heart Rhythm used advanced brain imaging to investigate the central nervous system in people with POTS. Researchers compared 18 people with POTS with 20 healthy controls. They used MRI and functional MRI to examine brain structure, cerebral blood flow and neural responses while participants underwent several challenges involving autonomic regulation. Those included passive leg raising, an isometric handgrip exercise and something particularly interesting: mental arithmetic. Researchers weren't only examining what happened during physical stress. They examined what happened while participants were thinking. What Did Researchers Find? General cognitive screening scores were similar between the POTS and control groups. That is an important finding by itself. People experiencing brain fog don't necessarily lose their overall intellectual abilities. However, when researchers looked at executive functioning, differences appeared. Participants with POTS were slower on a test involving executive function. Researchers also identified reduced cerebral blood flow, structural differences in certain brain regions and altered neural responses during the autonomic challenges. Those altered responses were observed during physical challenges—and during mental arithmetic. The authors suggested that abnormalities in central autonomic regulation could contribute to the cardiovascular dysregulation associated with POTS. This doesn't mean researchers have discovered the single cause of POTS brain fog. But it adds another important piece to the puzzle. What Is Cerebral Blood Flow? Cerebral blood flow simply refers to the blood circulating through the brain. Your brain requires a continuous supply of oxygen and nutrients, so the body has sophisticated mechanisms for regulating that circulation. Normally, cerebral circulation adapts as you stand, move, exercise, concentrate and encounter changes in blood pressure. Because POTS involves abnormal autonomic regulation, researchers have been investigating whether altered regulation of cerebral circulation might contribute to symptoms such as dizziness and cognitive dysfunction. And the 2026 study isn't the first to explore this connection. Earlier Research Found Something Fascinating About Mental Exertion In an earlier study, researchers examined people with POTS while they completed prolonged cognitive testing while seated. The researchers measured cerebral blood-flow velocity before and after approximately 30 minutes of sustained cognitive stress. Both the POTS group and healthy controls experienced some reduction in cerebral blood-flow velocity. But the decrease was significantly greater in the POTS group. The average reduction was approximately: 7.8% in the POTS group compared with 1.8% in healthy controls. The researchers also observed worsening cognitive performance. Perhaps most interestingly, the cerebral blood-flow reduction following prolonged cognitive stress in participants with POTS was comparable to what researchers observed after standing. That raises an intriguing possibility: Mental exertion itself may create meaningful physiological demands in POTS. Why Can Thinking Feel Physically Exhausting? We often divide activity into two categories. Physical activity is walking, standing, cleaning or exercising. Mental activity is reading, working, studying, planning or talking. Biologically, the distinction isn't nearly that simple. Your brain is part of your body. Concentrating requires neural activity, energy metabolism, oxygen delivery and regulation of cerebral circulation. For someone living with autonomic dysfunction, demanding cognitive activity may therefore represent a genuine physiological load. That could help explain an experience many people with chronic illness struggle to describe: “I didn't do anything today. Why am I so exhausted?” But perhaps you did. You worked. You studied. You navigated appointments. You processed information. You held conversations. You made decisions. You concentrated. Those activities may not involve many steps on a fitness tracker, but that doesn't mean they require no physiological resources. Thinking isn't physiologically free. Brain Fog Doesn't Mean You're Less Intelligent This distinction deserves more attention. The 2026 study found similar general cognitive screening scores between the POTS and control groups even though differences emerged in more specific measures of executive function. That may help explain why POTS brain fog can feel so strange. You may know that you know something while temporarily struggling to retrieve it. You may understand a complex subject but need substantially longer to process information. You may be capable of performing a task but unable to sustain that level of concentration for very long. Brain fog can involve difficulties with things such as: attention working memory processing speed word retrieval executive functioning sustained concentration mental stamina Those problems are not synonymous with intelligence. A highly capable person can experience significant cognitive dysfunction. Is All POTS Brain Fog Caused by Reduced Cerebral Blood Flow? No. This is where emerging research needs to be interpreted carefully. The 2026 neuroimaging study was small, involving only 18 participants with POTS. Earlier studies have also produced somewhat different results. For example, another small study found measurable problems with short-term memory and alertness in people with POTS but did not find significant differences in the particular cerebral blood-flow measurements researchers examined. That's not unusual in an evolving area of science. Different studies use different imaging techniques, cognitive tests, body positions, autonomic challenges and patient populations. POTS itself is also heterogeneous. There may not be one mechanism responsible for every person's cognitive symptoms. Brain fog could potentially be influenced by multiple factors, including autonomic dysfunction, cerebral circulation, sleep disruption, pain, medications, nutritional status, fatigue and coexisting medical conditions. The current research therefore doesn't establish: POTS brain fog = low blood flow to the brain. Instead, it tells us that cerebral circulation and central autonomic regulation are important areas of investigation. Why Can Brain Fog Happen Even While Sitting? This is another important question. It might seem logical that cognitive problems associated with POTS would disappear when someone sits down. But that isn't necessarily what people experience—or what researchers have observed. The sustained cognitive-stress study specifically examined participants while they were seated. Cerebral blood-flow velocity still declined more in the POTS group during prolonged mental work. That suggests cognitive symptoms can't necessarily be understood only as something that happens when a person stands. Posture matters enormously in POTS. But cognitive workload may matter too. Brain Fog May Also Change Throughout the Day Another reason brain fog can be difficult to understand is that it may not remain constant. A person might think clearly in the morning but struggle after several hours of activity. They may tolerate a short conversation but become cognitively depleted during a long appointment. They may read for ten minutes comfortably but struggle after an hour. That means one brief cognitive test in a clinical setting may not necessarily reproduce the difficulties someone experiences after sustained activity in everyday life. Researchers are beginning to study these dynamic responses rather than looking only at a single measurement taken at rest. That could be important for understanding conditions in which capacity changes with exertion. POTS Brain Fog and PEM Aren't Necessarily the Same Thing There is another distinction worth making. People with POTS may experience cognitive symptoms during orthostatic stress or prolonged mental activity. People with ME/CFS and some people with Long COVID may experience post-exertional malaise (PEM)—a broader worsening of symptoms following physical, cognitive, emotional or social exertion that can be delayed and may last for days or longer. Because these conditions can overlap, it can become difficult to determine what is causing a particular episode of cognitive exhaustion. Immediate difficulty thinking while upright isn't necessarily the same phenomenon as delayed PEM. Understanding that distinction is an important area for both research and patient education. What This Research Does Not Tell Us Emerging research can easily become exaggerated online, so several limitations are important. These studies do not currently show that: cerebral blood-flow testing can diagnose POTS brain fog everyone with POTS has reduced cerebral blood flow every episode of brain fog has the same cause brain imaging is routinely needed for POTS a particular supplement, medication or hydration strategy will correct cognitive dysfunction researchers have identified a single mechanism responsible for POTS brain fog The findings are clues. They are not a finished explanation. Why This Research Matters Anyway For a long time, POTS was frequently reduced to a simple description: “Your heart rate goes up when you stand.” But modern POTS research is revealing a considerably more complicated disorder involving interactions among the cardiovascular system, autonomic nervous system and brain. The newest research doesn't provide all the answers. What it does provide is something important: A reason to investigate brain fog as a physiological symptom rather than dismissing it as simply being tired, distracted or anxious. And that changes the question. Instead of asking: “Why can't I just concentrate?” Researchers can begin asking: “What happens to autonomic regulation, cerebral circulation and cognitive performance when a person with POTS has to think for an extended period of time?” That's a much more useful scientific question. And we're beginning to get answers. The Bottom Line Brain fog is a recognized and potentially disabling part of POTS. Emerging research suggests that cerebral blood flow and central autonomic regulation may contribute to cognitive dysfunction in at least some people with the condition. Researchers have also found something particularly interesting: sustained cognitive activity itself may produce measurable physiological changes in people with POTS. But the science is still developing. We don't yet have one explanation for POTS brain fog, and cerebral blood flow is unlikely to be the entire story. What the research increasingly demonstrates is that thinking, circulation and autonomic regulation are interconnected. For people living with POTS, that may help explain something they have understood intuitively for a long time: Mental activity is still activity. A Note From Complexa Wellness Complex chronic illness rarely fits neatly into one body system. Understanding the connections between the nervous system, circulation, energy demands, nutrition and everyday function can make complicated health information easier to navigate. At Complexa Wellness, our goal is to translate emerging chronic-illness research into accessible education—without turning early findings into miracle claims or pretending science knows more than it does. Wellness made simpler. This article is for educational purposes only and is not intended to diagnose, treat, cure or prevent any disease. Brain fog and new or worsening cognitive symptoms can have many causes. Discuss significant or changing symptoms with an appropriate healthcare professional. References Malik V, Roy B, Sarkar A, et al. Brain tissue changes, network dysfunction, and cerebral hemodynamic deficits in postural orthostatic tachycardia syndrome. Heart Rhythm. 2026;23(8):e1617-e1627. doi:10.1016/j.hrthm.2026.02.020. Chung TH, Raj SR. Postural Orthostatic Tachycardia Syndrome (POTS): A Review. JAMA. Published online August 24, 2026. doi:10.1001/jama.2026.14809. Wells R, Paterson F, Bacchi S, et al. Cerebral Blood Flow and Cognitive Performance in Postural Tachycardia Syndrome: Insights From Sustained Cognitive Stress Test. Journal of the American Heart Association. 2020. Anderson JW, Lambert EA, Sari CI, et al. Brain fog in postural tachycardia syndrome: An objective cerebral blood flow and neurocognitive analysis. Journal of Arrhythmia. 2020.

If you live with postural orthostatic tachycardia syndrome (POTS), you may know the feeling. You stand up and suddenly thinking becomes harder. You lose your train of thought halfway through a sentence. Words that should be easy to find disappear. Reading the same paragraph takes three tries. A conversation that would normally be effortless feels like work. Or perhaps the brain fog happens even when you aren't standing. For years, people with POTS have described problems with concentration, memory, processing speed and mental stamina. “Brain fog” has become the shorthand for all of it. Now researchers are getting a closer look at what may be happening inside the brain—and a 2026 study adds an intriguing piece to the puzzle. Brain Fog Is a Recognized Part of POTS POTS is usually defined by what happens to the cardiovascular system when a person becomes upright: heart rate rises abnormally and symptoms of orthostatic intolerance develop. But POTS can affect far more than heart rate. Fatigue, exercise intolerance, gastrointestinal symptoms, sleep problems and cognitive dysfunction are all commonly reported. A major 2026 review in JAMA includes cognitive dysfunction—often called brain fog—among the common symptoms associated with POTS. That matters because brain fog can be profoundly disruptive even though it is difficult to see from the outside. Someone may appear to be sitting still while their brain is struggling to keep up. A New Study Looked Directly at the Brain Researchers from UCLA and collaborating institutions recently used advanced brain imaging to investigate the central nervous system in people with POTS. The study, published online in February 2026 and appearing in the August 2026 issue of Heart Rhythm, included 38 participants: 18 people with POTS and 20 healthy controls. Researchers used MRI and functional MRI to examine brain structure, cerebral blood flow and brain responses during several challenges involving autonomic regulation. Those challenges included: Passive leg raising Isometric handgrip Mental arithmetic That last one is especially interesting. The researchers weren't only asking what happens to the brain during a physical cardiovascular challenge. They also looked at what happened while participants were thinking. The Results Were Interesting On a general cognitive screening test, the POTS and control groups performed similarly. But when researchers tested executive function—the collection of mental abilities involved in things such as organizing information, shifting attention, planning and completing complex tasks—the POTS group was slower. Researchers also found differences in brain structure in particular regions, reduced cerebral blood flow, and altered neural responses during the autonomic challenges. Those altered responses appeared during the physical challenges and during mental arithmetic. The researchers concluded that abnormalities involving central autonomic regulation may contribute to the cardiovascular dysregulation seen in POTS. That doesn't mean scientists have discovered the cause of POTS brain fog. But it does give researchers another physiological pathway to investigate. What Does Cerebral Blood Flow Have to Do With Brain Fog? Your brain is extraordinarily metabolically active. Although it represents only a small portion of your total body weight, it requires a continuous supply of oxygen and nutrients delivered through the circulation. Your body therefore has systems that continually regulate blood flow to the brain as you move, stand, exercise, think and encounter changes in blood pressure. POTS involves dysfunction of the autonomic nervous system—the system responsible for regulating many of the body's automatic processes. When you stand, gravity shifts blood downward. Normally, the cardiovascular and autonomic systems compensate rapidly enough to maintain circulation. In POTS, that adjustment doesn't work normally. Scientists have therefore been investigating whether abnormalities in cerebral circulation could contribute to symptoms such as dizziness, visual disturbances and cognitive dysfunction. This Isn't the First Study to Find a Connection The new research builds on earlier work. In one study of people with POTS undergoing prolonged cognitive testing while seated, researchers found that cerebral blood-flow velocity decreased more in the POTS group than in healthy controls. Their psychomotor speed also declined more. Remarkably, the reduction in cerebral blood-flow velocity following prolonged cognitive stress in the POTS group was similar to the reduction researchers observed after participants stood for five minutes. In other words, sustained mental work appeared capable of producing measurable physiological changes even without standing. Another study using SPECT brain imaging found abnormal cerebral perfusion in a substantial portion of the POTS patients with cognitive dysfunction who were studied. The research isn't perfectly consistent. A different small study found cognitive deficits without detecting significant differences in the particular cerebral blood-flow measurements it examined. That inconsistency is important. Science rarely progresses through one dramatic experiment that explains everything. Different researchers study different populations, use different imaging techniques and measure different parts of an enormously complicated system. Taken together, however, the research increasingly supports something people with POTS have been saying for years: Brain fog deserves to be studied as part of the physiology of POTS. Why Can Thinking Feel Physically Exhausting? This may be one of the most interesting questions raised by the research. We tend to divide activity into two categories: Physical activity: walking, standing, exercising, cleaning. Mental activity: reading, working, studying, planning, talking. Biologically, that distinction isn't so clean. Your brain is part of your body. Complex cognitive activity requires neural processing, energy, oxygen delivery, regulation of cerebral circulation and coordination between multiple physiological systems. For someone whose autonomic regulation is already under strain, demanding cognitive work may therefore represent a meaningful physiological load. That could help explain an experience frequently described by people with dysautonomia: “I didn't do anything today. I was just working on my computer. Why am I exhausted?” You did do something. You were thinking. And thinking isn't physiologically free. Brain Fog Isn't the Same Thing as Intelligence There's another important distinction. Cognitive dysfunction doesn't necessarily mean that someone's underlying intellectual ability has disappeared. In the 2026 study, general cognitive screening scores were similar between people with POTS and healthy controls. The differences became apparent when researchers examined more specific aspects of cognitive performance. That distinction may help explain another frustrating experience: Knowing that you know something while temporarily being unable to access or organize it efficiently. A person can be knowledgeable, capable and intelligent while simultaneously experiencing impaired processing speed, attention, working memory or executive function. Those things aren't contradictions. Does This Mean Every Case of POTS Brain Fog Is Caused by Low Brain Blood Flow? No. This is where we need to be careful. The 2026 neuroimaging study included only 18 participants with POTS. That's valuable research, but it's nowhere near enough to establish a universal explanation for millions of individual experiences. POTS is also heterogeneous. Brain fog could potentially be influenced by multiple factors, including autonomic dysfunction, cerebral circulation, sleep disruption, medications, pain, nutritional status, fatigue and coexisting illnesses. Different mechanisms may dominate in different people. Research showing an association between cerebral blood flow and cognitive performance therefore does not mean that every episode of brain fog is caused by inadequate blood flow to the brain. It also doesn't mean that brain imaging can currently diagnose POTS brain fog. These findings are clues—not a finished explanation. Why This Research Still Matters For someone living with POTS, being told that testing is “normal” can be incredibly confusing when everyday cognitive tasks have become noticeably harder. Studies like these demonstrate why researchers increasingly need to look beyond broad screening tests. A person can perform normally on one general cognitive assessment while experiencing meaningful difficulties with executive function, sustained cognitive exertion or autonomic regulation. The emerging picture is more complicated than: POTS = fast heart rate when standing. POTS involves autonomic regulation, and the autonomic nervous system interacts with circulation, digestion, temperature regulation, exercise responses—and the brain. Understanding those connections may eventually help researchers develop better ways to identify different POTS subtypes and determine why symptoms vary so dramatically between people. The Bigger Question: What Happens When the Brain Has to Work Hard? Perhaps the most interesting part of this research isn't simply that investigators found differences in people with POTS. It's when some of those differences appeared. Mental arithmetic was enough to produce altered neural responses in the new study. Earlier research found measurable cerebral blood-flow changes following prolonged cognitive stress. Together, those findings raise an important question for future research: Could cognitive exertion itself place a measurable physiological demand on an already dysregulated autonomic system? We don't have the complete answer yet. But it's a question worth asking—particularly for people who find that concentrating, studying, working at a computer, holding long conversations or processing large amounts of information can noticeably worsen their symptoms. The science of POTS is moving beyond simply measuring heart rate. Researchers are increasingly examining what happens throughout the body—and the brain. And that may eventually help explain why something as invisible as “brain fog” can have such a visible impact on everyday life. A Note From Complexa Wellness Complex chronic illness rarely fits neatly into a single body system. At Complexa Wellness, we believe people deserve accessible explanations of emerging research without hype, miracle claims or oversimplification. We follow developing research in POTS, dysautonomia, ME/CFS, Long COVID, mast-cell disorders, hypermobility and related chronic conditions and translate it into practical, understandable education. Wellness made simpler. This article is for educational purposes only and is not medical advice. New or worsening cognitive symptoms can have many causes and should be discussed with an appropriate healthcare professional. Sources Malik V, Roy B, et al. Brain tissue changes, network dysfunction, and cerebral hemodynamic deficits in postural orthostatic tachycardia syndrome. Heart Rhythm. 2026;23(8):e1617-e1627. doi:10.1016/j.hrthm.2026.02.020. Postural Orthostatic Tachycardia Syndrome (POTS): A Review. JAMA. Published August 24, 2026. Wells R, et al. Cerebral Blood Flow and Cognitive Performance in Postural Tachycardia Syndrome: Insights from Sustained Cognitive Stress Test. Novel brain SPECT imaging unravels abnormal cerebral perfusion in patients with postural orthostatic tachycardia syndrome and cognitive dysfunction. Brain fog in postural tachycardia syndrome: An objective cerebral blood flow and neurocognitive analysis.

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