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Chronic Fatigue · multi-factor + ME/CFS + Long COVID
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In one pass Tired is a symptom, not a diagnosis.
Educational content, not medical advice — consult a clinician.
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Chapter 1
'Tired' isn't a diagnosis
Tired is a symptom, not a diagnosis. The same words, I am so tired, can sit on top of completely different states of the body. Split it into four layers and you can tell whether you need a rest or a checkup.
Layer 1 · Ordinary tiredness: a short night, a hard workout, a stressful week at work. A night's sleep or a couple of days off brings you back. This layer does not point to any disease.
Layer 2 · Persistent tiredness (under 6 months): still tired after a full night's sleep, for several weeks in a row. There is usually a trigger, such as an acute illness, a stressful stretch or a change of season, and much of it clears up on its own.
Layer 3 · Chronic tiredness (over 6 months): once it has dragged on past half a year, it is rarely one cause. Sleep, nutrition, hormones, mood and chronic illness tend to stack up, and they need to be checked one by one. Many people end up finding at least one cause that can be corrected.
Layer 4 · ME/CFS (myalgic encephalomyelitis, also called chronic fatigue syndrome): a distinct illness, not tiredness, only worse. Its hallmark is a crash of the whole person after effort; the chapter ME/CFS and crashes after exertion is devoted to it.
Some situations must not be handled as mere tiredness. If tiredness comes with chest pain or severe shortness of breath, blood in the stool or black stools, or thoughts of harming yourself, seek care immediately. Unexplained weight loss, or night sweats with fever, also need prompt medical attention. The full list is in the chapter Checking tiredness step by step.
Layer 1 · Ordinary tiredness: a short night, a hard workout, a stressful week at work. A night's sleep or a couple of days off brings you back. This layer does not point to any disease.
Layer 2 · Persistent tiredness (under 6 months): still tired after a full night's sleep, for several weeks in a row. There is usually a trigger, such as an acute illness, a stressful stretch or a change of season, and much of it clears up on its own.
Layer 3 · Chronic tiredness (over 6 months): once it has dragged on past half a year, it is rarely one cause. Sleep, nutrition, hormones, mood and chronic illness tend to stack up, and they need to be checked one by one. Many people end up finding at least one cause that can be corrected.
Layer 4 · ME/CFS (myalgic encephalomyelitis, also called chronic fatigue syndrome): a distinct illness, not tiredness, only worse. Its hallmark is a crash of the whole person after effort; the chapter ME/CFS and crashes after exertion is devoted to it.
Some situations must not be handled as mere tiredness. If tiredness comes with chest pain or severe shortness of breath, blood in the stool or black stools, or thoughts of harming yourself, seek care immediately. Unexplained weight loss, or night sweats with fever, also need prompt medical attention. The full list is in the chapter Checking tiredness step by step.
Clinical · Why the last layer stands alone
The tiredness of the first three layers sits on a single axis, differing only in depth: spending outruns supply, and filling the gap brings you back. The last layer is not on that axis.At its core is not too little energy but the act of effort itself setting off a crash. Here it is called a crash after exertion (medicine calls it post-exertional malaise, PEM), and it is the sharpest line between this illness and the other kinds of tiredness.
Diagnosis rests on two similar sets of criteria: the US Institute of Medicine (IOM) criteria of 2015 and the 2021 guideline from England's National Institute for Health and Care Excellence (NICE). Both require that the symptoms cannot be explained by another illness; the chapter ME/CFS and crashes after exertion goes through the conditions one by one. Estimates of how common it is worldwide vary widely, and the figure most often quoted is about 0.4%. It hits daily functioning hard, and it overlaps heavily with Long COVID.
Notice the order: rule out first, then meet the criteria. Causes that can be tested for and treated, such as anemia, an underactive thyroid, sleep apnea and depression, have to be searched for properly before this diagnosis is even on the table. Skip that search and label yourself, and the biggest cost is not the label. It is missing a cause that could have been fixed.
Mechanism · Why supplements rarely fix tiredness
Tiredness is the complaint most often brushed aside. Attention-deficit/hyperactivity disorder (ADHD), anxiety, depression, an underactive thyroid, sleep apnea and iron deficiency can all be covered over by you are just tired. The shelf of anti-fatigue products on the market, such as the various adaptogens (herbs said to help the body cope with stress), high-dose B vitamins and injections, mostly does nothing about the root of the problem. The way out is to track the cause down layer by layer and treat that cause, not to pile supplements onto the body.Why so many problems come out through the same door
Because tired is not a reading from one organ. It is a single result that several lines in the body add up to: whether oxygen arrives, whether sugar gets into cells, whether sleep actually happened, whether the immune system has been firing for a long time, whether the drive to get going is still there. When any one line fails, what you feel is the same word.
That is also why the idea of topping up your energy so often fails. A supplement can add raw material, but most of these lines are not short of raw material; they are short of a channel, a rhythm or a signal. When iron is low, iron helps, because that really is a shortage of raw material. But sleep apnea is short of unbroken deep sleep, and an underactive thyroid is short of throttle, and no capsule supplies either.
So the chapter Common causes of tiredness is not a list of disease names but a map of what is short.
Chapter 2
Common causes of tiredness
Tiredness that has dragged on past half a year is usually not one cause but several stacked together. The more useful question is not which disease is this but which link in the body has failed. Grouped by what is short, the common causes fall into six sets:
Sleep that is not really sleep: enough hours in bed, but the quality stolen, as with sleep apnea, insomnia, restless legs or night shifts.A slack throttle: hormones turning the whole body's metabolic pace down, as with an underactive thyroid, poorly controlled blood glucose or falling sex hormones.Too few oxygen carriers: less oxygen reaching the tissues through the blood, as with iron deficiency or too little vitamin B12 or folate.The mind gave out first: the drive system stalls first, as with depression, anxiety, long-term burnout or attention problems.An organ quietly draining you: chronic disease of the kidneys, heart or liver, autoimmune disease, or a hidden chronic infection.A drug making you drowsy: some tiredness comes from medicines that directly dampen the nervous system.
Each set has its own texture of tiredness: when it is worst, what else the body is doing, and what makes it better. Those are clues you can pick out yourself and bring to your doctor.
Sleep that is not really sleep: enough hours in bed, but the quality stolen, as with sleep apnea, insomnia, restless legs or night shifts.A slack throttle: hormones turning the whole body's metabolic pace down, as with an underactive thyroid, poorly controlled blood glucose or falling sex hormones.Too few oxygen carriers: less oxygen reaching the tissues through the blood, as with iron deficiency or too little vitamin B12 or folate.The mind gave out first: the drive system stalls first, as with depression, anxiety, long-term burnout or attention problems.An organ quietly draining you: chronic disease of the kidneys, heart or liver, autoimmune disease, or a hidden chronic infection.A drug making you drowsy: some tiredness comes from medicines that directly dampen the nervous system.
Each set has its own texture of tiredness: when it is worst, what else the body is doing, and what makes it better. Those are clues you can pick out yourself and bring to your doctor.
Mechanism · Stolen sleep and a slack throttle
1. Sleep that is not really sleepThe lead suspect is obstructive sleep apnea (). After you fall asleep, the soft tissue of the throat loses its daytime tension and collapses, blocking the airway. Blood oxygen drops, and to prop the airway open again the brain has to wake you briefly. This micro-arousal is so short that you will not remember it the next day. Breathing resumes, you fall asleep, the airway collapses again, and this can repeat tens or even hundreds of times a night.
So what it steals is not hours but structure: deep sleep and dreaming sleep (rapid eye movement, or , sleep) are chopped into fragments. Deep sleep is when the body does repair work and consolidates the day's memories; once it is chopped up, the next day's tiredness is not I did not sleep but I slept and was never repaired. Anyone who is tired and snores, is heavy, or has high blood pressure should mention it to a doctor and consider being tested (see Obstructive Sleep Apnea).
The same set includes insomnia (trouble falling or staying asleep), restless legs (an indescribable discomfort in the legs at night and an urge to move them that pushes sleep later and later), and night shifts or jet lag scrambling the body clock. In that last case the hours may be entirely enough, but the sleep falls at a time the body clock does not recognize, hormone and temperature rhythms are out of step, and you wake as if you had not slept.
2. A slack throttle
Thyroid hormone works like the throttle for cells all over the body. It sets how fast cells burn nutrients into energy, and along the way how fast the heart beats, how fast the gut moves and where body temperature sits. In an underactive thyroid (hypothyroidism) the whole throttle is turned down, so it is not just tiredness but the whole body slowing together: feeling cold, constipation, a slow heart rate, dry skin, speech and thinking half a beat behind, weight creeping up (see Hashimoto's Thyroiditis).
The blood-glucose line works differently. With insulin resistance or diabetes, sugar stays in the blood and cannot get into cells. Some people get waves of sleepiness after meals, especially after refined carbohydrates. A common explanation is that glucose spikes, insulin floods out and overshoots, but after-meal sleepiness has more than one cause and this explanation has not been confirmed step by step. Falling testosterone in men and the hormone swings of menopause in women can also drag energy down through the hormone line (see Andropause and Perimenopause).
Mechanism · Short on oxygen, or the mind burns out
3. Too few oxygen carriersThe classic nutritional gap behind tiredness is iron deficiency. Iron is not the vague idea of building up the blood: it sits at the very center of hemoglobin and is the part that actually grips the oxygen molecule. When iron runs short, too little hemoglobin is made and each drop of blood carries less oxygen. Tissues that cannot get enough oxygen force the heart to beat faster and the breathing to quicken, making up with flow what the blood lacks in content.
That gives this tiredness a recognizable texture. Sitting still may be fine, but climbing stairs or lifting something shows it at once, with a racing heart and breathlessness arriving sooner than they used to. It often comes with a pale face and pale inner eyelids, brittle nails and hair loss, and some people even develop a strange urge to chew ice. It is especially common in women with heavy periods and in vegetarians.
Too little vitamin B12 or folate both lead to badly made red cells (larger and fewer). A lack of B12 also impairs the upkeep of myelin, the insulating layer around nerves, so the typical pair in B12 deficiency is tired and numb: pins and needles in the hands and feet and an unsteady walk. Tiredness plus numbness deserves its own alarm (see Vitamin B12).
Vitamin D deficiency is also a regular item on the checklist. And when older people, or people dieting, eat too little protein, muscle quietly wastes away. With less muscle, the same walk uses a larger share of your strength reserve, so the same task takes more out of you.
4. The mind gave out first
The tiredness of depression is not a flat battery but being unable to start. Physical tiredness eases when you lie down; the tiredness of depression does not, and it often comes with losing interest in everything, constant self-blame and waking before dawn.
Anxiety runs the other way. The sympathetic nervous system (the body's fight-or-flight system) is braced all day, like an engine idling at high revs. The comparison makes only one point: staying braced is itself exhausting, and at night it scrambles sleep as well. Long periods of high pressure, and the burnout that comes from caring for family, really do wring energy out of you. One more that is easily missed: attention problems (ADHD) can also pass for just being tired, because forcing your attention to stay on one thing is like doing a full day's labor.
Mechanism · Organs draining you, and drowsy drugs
5. An organ quietly draining youWhen organs that work in the background, such as the kidneys, heart and liver, develop chronic problems, the early sign is often a tiredness you cannot put a name to. Take the kidney. Besides clearing waste water, it sends the order to make blood, a hormone called erythropoietin (EPO). In chronic kidney disease that order weakens, fewer red cells are made, and you are back in the oxygen-carrier set, so two lines can fail in the same person. Heart failure is a pump that cannot keep up: tissues are underfed with blood and any exertion brings breathlessness. With a liver problem, the hub for metabolism and detoxification is running at a discount.
Autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus and Hashimoto's thyroiditis, and hidden infections such as chronic hepatitis B, HIV and Lyme disease, also make people tired, and the tiredness has a particular feel. The immune system has been firing for a long time, and the inflammatory signals it releases act on the brain, producing a heavy urge to curl up and stay still. The two days of a fever when all you want is to lie down are a short version of the same reaction. This set often comes with other clues: joint pain, rashes, recurring low fevers, swollen lymph nodes.
If this tiredness is new and comes with unexplained weight loss or night sweats, that is a red flag: seek care promptly so that cancer can be looked for properly.
6. A drug making you drowsy
Some tiredness comes from medicines. Older allergy antihistamines (diphenhydramine, sold in the US as Benadryl, for example) cross the blood-brain barrier and block the histamine signal in the brain that keeps you awake, so they make you drowsy outright. Benzodiazepines taken for sleep or anxiety amplify the brain's main braking signal, and besides sedating you they reduce the share of deep sleep. Opioid painkillers damp down the central nervous system as a whole. Some blood-pressure drugs (beta-blockers in particular) lower heart rate and exercise capacity together, so that two flights of stairs become too much.
Regular heavy drinking belongs here too, and it is deceptive. In the first half of the night alcohol acts like a sedative and you fall asleep fast; in the second half, as the alcohol is broken down, sleep turns shallow and you wake again and again. The hours look sufficient, but the repair work was never finished.
Clinical · How the common kinds of tired differ
The same words, I am so tired, can sit on quite different feelings. Pay attention to three things: when it is worst, what else your body is doing at the time, and what makes it better. That alone hands your doctor half the clues.Iron-deficiency tiredness: fine at rest, but it shows up with any effort. Stairs, carrying bags or a few quick steps set the heart racing and leave you breathless. It comes with a pale face and pale inner eyelids, brittle nails and hair loss. A short rest eases it, and the next effort brings it back.Underactive-thyroid tiredness: an even, all-day dullness that no amount of sleep improves, with the whole body slowing down: feeling cold, constipation, a slow heart rate, dry skin, slow reactions. It keeps no particular hours and does not rise and fall much with activity.Sleep-apnea tiredness: enough hours of sleep, yet daytime sleepiness you cannot fight off, dozing in meetings, in front of the TV or at a red light. Often there is a dry mouth on waking, a morning headache and frequent trips to the toilet at night. The most valuable clue comes from the person next to you: snoring, and pauses in breathing they have noticed.Depression tiredness: not an inability to exert force but an inability to summon the will. Lying down does not ease it; it is often worst in the morning and comes with losing interest in things you used to enjoy, self-blame and early waking.After-meal tiredness: it follows meals, with a stretch of drowsiness especially after refined carbohydrates, and the rest of the day is fine.
This is not so that you can diagnose yourself. These can exist together, and they often do. The point is to describe tiredness as a concrete sequence of events rather than an adjective, which makes the doctor's first round of screening far more accurate.
Clinical · How much one basic blood draw can screen out
Do not let the long list alarm you. At the doctor's, one basic blood draw usually screens most of it: a (to look for anemia), liver and kidney function, thyroid (thyroid-stimulating hormone, ), , B12 and folate, vitamin D (a blood below 50 nmol/L, which is 20 ng/mL, counts as low, and deficiency is further down), (HbA1c, which reflects average blood glucose over the past two to three months), plus one inflammation marker and a few short questionnaires screening for depression, anxiety, insomnia and snoring. For people over 40, or anyone with red flags such as weight loss or night sweats, the doctor may add prostate, hormone or cancer-related tests as needed.Each item answers a very specific question:
The blood count asks is there enough blood, and are the red cells built right; ferritin goes a step further and asks how much iron is left in the store. The blood count can still be normal while the store is nearly empty, and this is exactly where iron deficiency is most often missed.The thyroid test asks how loudly the brain is urging the thyroid to work: the louder the urging, the less thyroid hormone the rest of the body usually has.HbA1c shows your average blood glucose over a recent stretch, not this morning's reading.An inflammation marker does not tell you which disease you have, only whether something in the body has been firing for a long time, which is worth following up.The questionnaires are not a formality: depression, anxiety, insomnia and snoring are exactly the common causes that a blood test cannot see.
The point is not to memorize the abbreviations but to grasp one thing: tiredness can be worked through systematically. Do not rush to diagnose yourself as short of some nutrient.
Chapter 3
ME/CFS and crashes after exertion
ME/CFS (myalgic encephalomyelitis, also called chronic fatigue syndrome) is a distinct illness, unlike any of the tiredness above. What sets it apart is the crash after exertion (medicine calls it post-exertional malaise, PEM).
PEM comes first because it is the key to understanding the illness. It is not tired, so rest and it passes. It is you did something, and a day or two, even several days, later, the whole person collapses. The trigger can be very light: a ten-minute walk, a shower, seeing a friend, and mental and emotional effort count just as much. It is also often delayed: you may feel fine at the time and crash, typically, 24–72 hours later.
The cleanest difference from ordinary tiredness fits in one sentence: ordinary tiredness goes away after good sleep; ME/CFS does not, and effort makes it worse.
So people with ME/CFS have to learn to stay within their daily energy envelope: work out how much they can spend today and do not overdraw, because going over can set off the next crash. Planning activity around that limit, spending only what the budget allows, is called pacing, and it is the core of management today.
PEM comes first because it is the key to understanding the illness. It is not tired, so rest and it passes. It is you did something, and a day or two, even several days, later, the whole person collapses. The trigger can be very light: a ten-minute walk, a shower, seeing a friend, and mental and emotional effort count just as much. It is also often delayed: you may feel fine at the time and crash, typically, 24–72 hours later.
The cleanest difference from ordinary tiredness fits in one sentence: ordinary tiredness goes away after good sleep; ME/CFS does not, and effort makes it worse.
So people with ME/CFS have to learn to stay within their daily energy envelope: work out how much they can spend today and do not overdraw, because going over can set off the next crash. Planning activity around that limit, spending only what the budget allows, is called pacing, and it is the core of management today.
Clinical · Rule out other causes, then check criteria
How it is diagnosedThe US Institute of Medicine (IOM) criteria of 2015 require three core features, plus at least one more.
All three of these: a marked drop in function lasting more than 6 months, with tiredness that rest does not relieve; PEM (symptoms that worsen after effort, often lasting more than 24 hours, and for some people 2–7 days); and sleep that does not refresh.Plus at least one of these: brain fog (slowed thinking, poor memory) or orthostatic intolerance (heart rate and blood pressure going wrong on standing up, easing when lying down).The symptoms must be present at least half the time, at moderate intensity or worse, for the diagnosis to hold.
The 2021 guideline from England's National Institute for Health and Care Excellence (NICE) uses a similar set: debilitating fatigue, PEM, unrefreshing sleep and brain fog must all be present, and once they have lasted 3 months and cannot be explained by another illness, the diagnosis can be made. Under neither set does any single lab test settle it.
The shape of these criteria is worth a second look. Because there is no decisive lab test, every item describes how function has changed rather than what a number is: less you can do, a crash after effort, sleep that does not refresh, a mind that cannot keep up, a body that cannot stay upright. That is why, at an appointment, describing one day in concrete detail is more useful than a stack of lab reports.
One more point is often read backward: ruling out first is not optional. The IOM asks for the diagnosis to be made only after a thorough history, physical examination and targeted tests, precisely to rule out other illnesses that could explain the symptoms and to find any that exist alongside it. Causes that can be tested for and treated, such as anemia, an underactive thyroid, sleep apnea and depression, have to be searched for properly first.
Numbers · How many have it, and why it was ignored
Estimates of how common it is worldwide vary widely; the figure most often quoted is about 0.4%. The 2015 IOM report estimated that in the United States alone somewhere between 840,000 and two and a half million people have it, and that an estimated 84–91% of them have not been diagnosed.It was neglected for decades. Between 1970 and 2000 it was dismissed as yuppie flu or treated as a purely psychological problem. Only the 2015 IOM report stated clearly that it is a serious, chronic, body-wide physical illness.
Why could an illness affecting so many people be misread for so long? Because it gathers together the things most easily dismissed as subjective: the complaint is a feeling (tiredness), the physical signs are nearly invisible, and routine lab tests are mostly normal. Medicine has never been kind to suffering that tests cannot find, and this illness falls into every one of those traps.
For readers, this history has a practical use. If you or someone close to you has been sent away with your tests are all normal, you are overthinking it, that sentence carries far less weight than these diagnostic criteria.
Clinical · Why just move more makes it worse
One counterintuitive but important fact: NICE 2021 no longer recommends graded exercise therapy (GET, a program that raises the amount of exercise in fixed steps) and says explicitly not to offer it for ME/CFS. The reason is that in this illness, the step-by-step increase itself can set off PEM again and again and make the illness worse. That is why well-meant advice like just move more and you will feel better is wrong for this illness.The difference is this. When ordinary people lose fitness from sitting too much, gradual increases really do train it back. The problem in this illness is not too little training but going over the limit sets off a crash, so the same step-up plan amounts to leaning on the trigger.
There is still no cure. The first line is pacing and staying within the energy envelope; beyond that, specific problems such as insomnia, pain and postural orthostatic tachycardia syndrome (POTS, in which heart rate shoots up on standing) are treated as they come. Treatments still in trials include low-dose naltrexone (LDN), antivirals and work on the gut microbiome; rituximab was dropped as a candidate treatment in 2018. There is no evidence that B-vitamin injections, infusions or mixed adaptogen products treat this illness.
Nutrition and supplements can help only so much. NICE 2021 is blunt: there is not enough evidence to support routinely taking vitamin and mineral supplements to treat ME/CFS or relieve its symptoms. A real deficiency should be corrected, and people who are bedbound or rarely go out should watch especially for low vitamin D. One review notes early signals for and D-ribose in small studies (Davis 2023), and people often try B12 and magnesium too. If you want to try one, talk through the possible side effects with your doctor first, and do not mistake ME/CFS for a missing nutrient; it is not that easy to fix.
In practice · Telling PEM from overtraining
Overdoing a workout and PEM look a little alike, but they differ in three places.Timing: ordinary soreness after exercise is worst the same day or the next and then steadily improves. PEM often arrives a day or two late, and what arrives is not just soreness: the whole person's functioning drops at once.Threshold: overdoing it assumes you did a lot. PEM can be set off by walking to the corner shop, taking a shower or sitting through a meeting. Mental and emotional effort count just as much, and this is what outsiders most often misunderstand.Cost: soreness is paid off with a couple of days' rest. Recovery from PEM is often out of all proportion; one overdraft can take several days or longer to repay, and until it is repaid, the ceiling on what you can do sits lower.
So pacing is not moving a bit less. It is measuring your limit first and then always keeping a margin. In practice it usually looks like this: break activity into short pieces and lie down on purpose between them, even if you feel you could keep going; record which activities were followed by a crash and how long after, and slowly map your ceiling; and on a day when you feel especially good, hold back hardest, because that is exactly when it is easiest to overspend and be knocked back to where you started.
For family and colleagues, the most useful sentence is this: being able to do something today does not mean being able to do it tomorrow. It is not a question of willpower; it is a question of budget.
Chapter 4
Why long COVID affects many organs
If ME/CFS was once a niche illness, Long COVID is the large patch that spread suddenly after 2020. Four weeks after recovering from acute COVID, some people develop new, lingering or recurring symptoms that last more than 12 weeks and have no other explanation. Medicine also calls it post-acute sequelae of SARS-CoV-2 infection (PASC).
The key point is that it does not stay in the lungs. To get into a cell, the virus first has to fit a receptor on the cell surface called ACE2 (think of it as a lock that the virus's key happens to fit), and that lock is not found only in the lungs: the lining of blood vessels, heart muscle, the gut and the nose all carry it. So from the start this was never an infection confined to the airways, and that is one explanation for why the after-effects are scattered across several body systems.
Under the same name, two people can look nothing alike: one has palpitations and cannot stay on their feet, another has brain fog, another is short of breath, another has gut problems.
The key point is that it does not stay in the lungs. To get into a cell, the virus first has to fit a receptor on the cell surface called ACE2 (think of it as a lock that the virus's key happens to fit), and that lock is not found only in the lungs: the lining of blood vessels, heart muscle, the gut and the nose all carry it. So from the start this was never an infection confined to the airways, and that is one explanation for why the after-effects are scattered across several body systems.
Under the same name, two people can look nothing alike: one has palpitations and cannot stay on their feet, another has brain fog, another is short of breath, another has gut problems.
Clinical · Where in the body it shows up
In the body it falls roughly into these patches:Heart and circulation: palpitations, chest pain, a heart rate that races for no clear reason. Postural orthostatic tachycardia syndrome (POTS, in which heart rate shoots up on standing) has become clearly more common, and it can follow even a mild case.Brain: brain fog, poor memory, scattered attention, headaches. A UK Biobank study that scanned people's brains once before and once after infection (Douaud 2022) found that, compared with people who had not been infected, those who had showed more thinning of gray matter in some smell-related brain regions and slightly more shrinkage of the brain overall. This is an observed association, most participants had mild cases, and what these changes mean for daily life is not yet clear. Many people are getting their first real taste of a brain that will not turn over.Breathing: shortness of breath, a chronic cough, no stamina after the slightest effort.Fatigue and PEM: this patch overlaps directly with ME/CFS. It is not not yet recovered but the genuine crash after exertion seen in ME/CFS.Autonomic nervous system: a heart rate that swings high and low, sweating and temperature control thrown into disorder.Gut: diarrhea, bloating, a changed appetite; some people newly develop irritable bowel syndrome.Other: joint pain, rashes, menstrual disturbances, smell and taste that do not come back.
POTS is the one most worth taking apart, because it is so easily read as a weak constitution. The moment you stand up, a good share of your blood sinks toward the lower body under gravity. Normally the autonomic nervous system tightens the leg vessels at once and squeezes that blood back to the heart. When that reflex is sluggish, too little blood returns to the heart, and the heart has to pound to keep up the supply to the brain. So sitting is fine, but on standing come palpitations, dizziness, blacking out and an empty head. It is not that the person does not want to stand; standing itself is using up a large share of the heart's output.
Mechanism · How each hypothesis would explain fatigue
Plenty of people end up here. The Davis 2023 review estimates that about 10–30% of people with acute COVID who were not hospitalized go on to Long COVID (more often women), and that at least about 65 million people worldwide already have it.As for why, the current view is that several mechanisms operate together rather than one cause: fragments of the virus may linger in tissues and never be fully cleared; the immune system is thrown off and starts making antibodies against the body's own tissues; microclots form in the smallest vessels and the vessel lining is damaged; the gut microbiome falls out of balance; herpes viruses lying dormant in the body (such as Epstein-Barr virus and human herpesvirus 6) are woken again; and signaling in the brainstem and the vagus nerve goes wrong.
These sound like separate stories, but if the hypotheses hold, they meet at one point: why you are tired. Oxygen and nutrients are handed over to tissues in the very finest layer of vessels. A damaged lining plus microclots narrows that last delivery point, so a blood count can be entirely normal while the far ends of the tissue still do not get enough. A long-firing immune system is the other line: inflammatory signals themselves make you not want to move, the same reaction as the two days of a fever when all you want is to lie down, only stretched over months.
Mind the word hypothesis: every item above is still being tested. That is exactly why any treatment that claims to clear one of them is, for now, running ahead of the evidence.
Clinical · The overlap with chronic fatigue syndrome
The fact most worth keeping is the overlap with ME/CFS. By the Davis 2023 review's estimate, about half of people with Long COVID meet the diagnostic criteria for ME/CFS, sharing PEM, unrefreshing sleep, orthostatic intolerance and brain fog. With so many new patients arriving at once, funding for Long COVID research rose too (the US NIH's RECOVER program alone put in about $1.6 billion), and ME/CFS, an illness neglected for decades, was pushed back in front of researchers along with it.This overlap has a very practical consequence. If your Long COVID includes PEM, then the pacing and energy envelope described in the chapter ME/CFS and crashes after exertion apply to you now; you do not need to wait for a new name. By the same logic, graded step-up exercise should be avoided here as well.
It also rewrote a default. Tiredness that tests cannot find used to be put down to psychology. Long COVID sent large numbers of previously healthy people, who can name the exact date it began, into the same clinics, and the default of doubting the patient first has become harder and harder to defend.
In practice · What helps now, and where not to bet
There is still no approved drug specifically for Long COVID. Management shares its roots with ME/CFS: pacing to protect energy; for POTS, more fluids, somewhat more salt and compression stockings (with medication if needed; anyone with high blood pressure or kidney disease should ask a doctor before adding salt); and treating headaches, insomnia, mood and gut problems as they come.On prevention: people who were vaccinated are less likely to develop Long COVID after infection (Davis 2023 review). Whether taking nirmatrelvir/ritonavir (Paxlovid) during the acute illness reduces Long COVID is so far mainly a signal from observational studies and still needs randomized trials to confirm it.
Once you follow the POTS mechanism above, salt, fluids and compression stockings stop looking like folk remedies. More water and salt hold blood volume up, and the stocking squeezes blood back up from the legs from the outside, standing in for a sluggish vessel reflex. Both fix the same link: the amount of blood that returns to the heart the moment you stand.
As for treatments claiming to detox you or chelate heavy metals out, cocktails of hyperbaric oxygen plus plus stem cells, and herbal antiviral formulas, most have no decent controlled trial behind them, so do not stake your hopes on them. The sound path is a specialist Long COVID clinic, a clinical trial if appropriate, plus symptom treatment and pacing.
Chapter 5
Checking tiredness step by step
When you are tired, check yourself at home for two weeks first, then see a doctor for a systematic workup. The first step is always to look for red flags.
Week 1 · Self-check and red flags
Red flags (seek care immediately)
Unexplained weight loss of more than 5% in 6 monthsNight sweats with fever and swollen lymph nodesA new, severe headache or new neurological symptomsChest pain or severe shortness of breathBlood in the stool or black stoolsAny tiredness with depression and thoughts of self-harm → get mental-health or psychiatric care immediatelyTiredness, and a partner has seen your breathing stop: get assessed for as soon as possible
The first item means losing more than 5% of your body weight over half a year for no clear reason. OSA is obstructive sleep apnea. If there are no red flags, move on to week 2.
Week 2 · Go through your lifestyle
Sleep: how long, how well, any snoring, the bedroom, screens before bedExercise: too much (overtraining), or too little and no strength trainingDiet: enough protein, enough iron, when you eat, how much processed foodAlcohol, caffeine and the medicines you takeStress, work and relationships
Week 1 · Self-check and red flags
Red flags (seek care immediately)
Unexplained weight loss of more than 5% in 6 monthsNight sweats with fever and swollen lymph nodesA new, severe headache or new neurological symptomsChest pain or severe shortness of breathBlood in the stool or black stoolsAny tiredness with depression and thoughts of self-harm → get mental-health or psychiatric care immediatelyTiredness, and a partner has seen your breathing stop: get assessed for as soon as possible
The first item means losing more than 5% of your body weight over half a year for no clear reason. OSA is obstructive sleep apnea. If there are no red flags, move on to week 2.
Week 2 · Go through your lifestyle
Sleep: how long, how well, any snoring, the bedroom, screens before bedExercise: too much (overtraining), or too little and no strength trainingDiet: enough protein, enough iron, when you eat, how much processed foodAlcohol, caffeine and the medicines you takeStress, work and relationships
Clinical · Tests and treatment in months 1–6
Weeks 3–4 · A systematic workup with your family doctorFirst-round tests (the chapter Common causes of tiredness explains what each one asks): , liver and kidney function with electrolytes, thyroid-stimulating hormone (), , B12, folate, vitamin D (), (HbA1c) and C-reactive protein (, an inflammation marker), plus a few short questionnaires: depression (PHQ-9), anxiety (GAD-7), insomnia, and the STOP-BANG questionnaire that screens for sleep apnea.Men over 40: depending on the case, add (PSA), a morning testosterone level, and and (LH/FSH).Women of childbearing age: add a pregnancy test (hCG), and a gynecological assessment if needed.
Months 1–2 · Find the reversible causes and treat them
Sleep apnea: continuous positive airway pressure (, a mask worn during sleep that holds the airway open with airflow) (see Obstructive Sleep Apnea)Underactive thyroid: levothyroxine (see Hashimoto's Thyroiditis)Iron or B12 deficiency: correct itDepression: talking therapy, with an antidepressant if needed, such as a selective serotonin reuptake inhibitor ()Insomnia: cognitive behavioral therapy for insomnia () (see Insomnia)Low vitamin D: correct itExisting chronic illness: get it under control
Months 3–6 · Check for improvement
Many people improve clearly once 1 or more causes have been found and dealt withStill no better: referral to a chronic fatigue, Long COVID or ME/CFS clinic
Whether there is PEM decides where to go next
The key question: after effort, is there a delayed crash (PEM)?
Yes: assess against the ME/CFS criteria, start pacing, and refer to an ME/CFS or Long COVID clinicNo: keep looking for other causes
In practice · Telling the doctor about your tiredness
Appointments are usually short, and I have been really tired lately carries almost no information. Tell the same thing as a sequence of events with a shape, and the doctor's first round of screening becomes much more accurate. These are the useful parts:When it started: did it creep up, or can you point to a particular week, an illness or a change of medicine? Tiredness with a clear starting point narrows the search at once.How it moves through the day: worst in the morning, or heavier as the evening goes on? After waking, is it better or the same? Not feeling better after sleep points to the quality of sleep, not the quantity.What makes it worse: after effort, are you tired the same day, or do you crash a day or two later? (Say the second out loud; it is the key clue to PEM.) Does it follow meals?What else is going on: feeling cold, constipation, a racing heart, breathlessness, pins and needles in the hands and feet, headaches, a change in weight, a change in menstrual flow. These by the way details are often the ones that set the direction.What other people have noticed: snoring, pauses in breathing, dozing off during the day without meaning to. You cannot supply this one yourself, but it carries a lot of weight.What you are taking: prescriptions, over-the-counter medicines, supplements, alcohol. Bring the boxes; they are more reliable than names from memory.
One last thing. If the doctor says the tests are all normal, that means this round of tests did not find it, not you are fine. You can follow up with: so which direction should we look next? That question is usually more useful than asking for another blood test.
Myth · What anti-fatigue products rest on
To dig deeper into a particular causeSleep apnea: one of the causes of chronic fatigue most often missed (see Obstructive Sleep Apnea)Insomnia: the different kinds, and why trying harder to sleep makes it worse (see Insomnia)Hashimoto's thyroiditis: a common source of an underactive thyroid (see Hashimoto's Thyroiditis)Iron: how red cells carry oxygen, and where iron-deficiency anemia comes fromVitamin B12: how a deficiency affects red cells and nerves (see Vitamin B12)Magnesium: why , the cell's energy molecule, depends on magnesiumEndocrine System: metabolic syndrome, insulin resistance and after-meal sleepinessAndropause and Perimenopause: hormone-related fatigue
Where the evidence cannot carry the marketing
, and (precursors sold as a way to recharge your cells): no randomized trial has shown that they treat fatigueMixed adaptogen products (ginseng, rhodiola, maca, ashwagandha): the evidence is mostly small, short trials that cannot support anti-fatigue claims, and they are no substitute for finding the causeIV drips (vitamin C, B vitamins, glutathione, the so-called Myers' cocktail): $200-500 a session in the US, heavily marketed, weak evidenceHyperbaric oxygen: a few small randomized, sham-controlled trials do report positive results (for example, Hadanny 2024's one-year follow-up, which retained only 31 participants), but all of them enroll dozens of people and they come mostly from a handful of centers; independent replication is still neededHeavy-metal chelation, parasite detoxes and candida detoxes: no evidence, and real risksAdrenal cocktails and other adrenal fatigue treatments: adrenal fatigue is not a recognized medical diagnosis
Tiredness is a symptom, not a diagnosis. Many cases of chronic fatigue have causes that can be identified and corrected. Finding the cause and dealing with it matters more than any bottle of anti-fatigue supplements.
References · 6
- Institute of Medicine (US) Committee on the Diagnostic Criteria for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. (2015). Beyond myalgic encephalomyelitis/chronic fatigue syndrome: redefining an illness. National Academies Press. 10.17226/19012
- National Institute for Health and Care Excellence. (2021). Myalgic encephalomyelitis (or encephalopathy)/chronic fatigue syndrome: diagnosis and management (NG206). NICE. www.nice.org.uk/guidance/ng206
- Benjafield, A. V., Ayas, N. T., Eastwood, P. R., Heinzer, R., Ip, M. S. M., Morrell, M. J., et al. (2019). Estimation of the global prevalence and burden of obstructive sleep apnoea: a literature-based analysis. The Lancet Respiratory Medicine, 7(8), 687-698. 10.1016/S2213-2600(19)30198-5
- Davis, H. E., McCorkell, L., Vogel, J. M., & Topol, E. J. (2023). Long COVID: major findings, mechanisms and recommendations. Nature Reviews Microbiology, 21(3), 133-146. 10.1038/s41579-022-00846-2
- Douaud, G., Lee, S., Alfaro-Almagro, F., Arthofer, C., Wang, C., McCarthy, P., et al. (2022). SARS-CoV-2 is associated with changes in brain structure in UK Biobank. Nature, 604(7907), 697-707. 10.1038/s41586-022-04569-5
- Hadanny, A., Zilberman-Itskovich, S., Catalogna, M., Elman-Shina, K., Lang, E., Finci, S., et al. (2024). Long term outcomes of hyperbaric oxygen therapy in post covid condition: longitudinal follow-up of a randomized controlled trial. Scientific Reports, 14, 3604. 10.1038/s41598-024-53091-3