- 01Tired, sleepy and weak are three different complaints
Fatigue is a lasting lack of energy that rest does not clear. Sleepiness is the pull to doze off, and it does improve with sleep, so it points toward sleep itself (too little, or broken by sleep apnea). Weakness means muscles that cannot do what they used to. Naming the right one sends you and your clinician to the right causes.
- 02Normal blood work narrows the search; it does not end it
Several of the commonest causes of constant tiredness (poor sleep, sleep apnea, stress and low mood) never show up on a blood test. A normal panel rules out several treatable causes, and a low-normal ferritin or B12 can still be worth a second look with your clinician.
If you are tired all the time, the cause is most often something in daily life: too little or poor-quality sleep, stress, depression or anxiety, alcohol or late caffeine, too little activity, or a medicine such as an antihistamine or sleeping pill. 23 The medical causes clinicians check most often are sleep apnea (breathing that stops and starts in sleep), iron deficiency and anemia (too few healthy red blood cells), an underactive thyroid, diabetes and low vitamin B12. 134 Fatigue is one of the top 10 reasons for a primary care visit, so you are far from alone. 1
When to get help now
Most tiredness is not dangerous. These signs are the exceptions.
- Chest, arm or upper back pain, or a racing or irregular heartbeat, with the tiredness 6
- Shortness of breath at rest or with very little effort 6
- Fainting, or new confusion or dizziness 3
- Black, tarry stools, vomiting blood or material like coffee grounds, or a lot of blood in your stool 13
- Sudden weakness or numbness on one side, a drooping face or trouble speaking 14
- Thoughts of harming yourself: call or text 988, the Suicide & Crisis Lifeline 5
- Small amounts of blood in your stool 13
- Weight loss you can't explain, fevers or regular sweats 3
- Little or no urine, or new swelling with weight gain 3
- Loud snoring with gasping or choking in sleep, especially if you drive drowsy 24
- Feeling low or hopeless most days 5
- Tiredness for a few weeks with no clear reason, or that stops you doing daily things 2
LifeFrom can't examine you or assess a symptom. If something feels seriously wrong, trust that and get care rather than waiting on a test.
Tired, sleepy or weak: which one is it?
Everyone gets tired, and the NHS says “It's normal to feel tired sometimes.” 2 The word covers three different complaints, and doctors and patients mix them up. 1 MedlinePlus defines fatigue as “a feeling of weariness, tiredness, or lack of energy,” and drowsiness as “feeling the need to sleep.” 3 The difference matters because sleepiness, the tendency to doze off, improves with rest, while fatigue often does not. 1 Weakness is a third thing: muscles that cannot lift, climb or grip the way they did.
| What you feel | What it usually points to first | Who checks it |
|---|---|---|
| Sleepy: dozing off in meetings, on the couch or at the wheel | Too little sleep, broken sleep, sleep apnea, a sedating medicine 34 | A clinician, sometimes with a sleep study |
| Tired: no energy or drive, even after a full night | Stress, low mood, anemia, thyroid, blood sugar, recovering from an infection 13 | A clinician, often with blood tests |
| Weak: muscles give out, trouble climbing stairs or gripping | Muscle, nerve or salt problems; sudden one-sided weakness is a stroke sign 14 | A clinician soon; call 911 if it is sudden |
Clinicians also sort fatigue by how long it has lasted. Fatigue lasting six months or longer is called chronic. 1 A few days of tiredness after a late week is not a symptom to chase; a few weeks with no reason you can name is. 2
What can cause it, most likely first
Ranked from most to least likely in adults in general. The order shifts with age, sex and history: iron deficiency moves up for women with heavy periods, thyroid problems for women in midlife, and diabetes and B12 for older adults or people on certain medicines.
- 01Too little sleep, or sleep that doesn't restore you
Poor sleep or trouble falling asleep is the first cause the NHS lists, and it suggests aiming for 6 to 9 hours. 2 Shift work, a new baby, late screens and an irregular bedtime all cut into it. If you feel sleepy rather than drained, start here; see can't sleep for what keeps people awake.
- 02Alcohol, caffeine, inactivity and eating patterns
An unhealthy diet, low activity and heavy drinking can all leave you tired, and caffeine or alcohol in the hours before bed disturbs sleep. 2 Cleveland Clinic lists diet, alcohol and drug use, inactivity, burnout and jet lag among the lifestyle factors. 6 Alcohol and drug use are on the MedlinePlus list of common causes too. 3
- 03Medicines
Antihistamines, blood pressure medicines, sleeping pills, steroids and diuretics (water pills) can cause drowsiness or fatigue, and so can sedatives and antidepressants. 3 Tiredness that started soon after a new medicine or a dose change is a strong clue. Do not stop a medicine on your own; ask the prescriber whether it could be the cause.
- 04Stress, depression and anxiety
Stress, depression and big life events such as a bereavement are common causes of tiredness. 2 MedlinePlus lists “Fatigue and lack of energy,” trouble concentrating and sleeping too much or too little among the signs of depression. 5 In one study, nearly 17% of people who saw a clinician for fatigue received a psychological diagnosis. 1 No blood test checks this; a conversation does. If low mood comes with thoughts of harming yourself, call or text 988. 5
- 05Sleep apnea
In sleep apnea, breathing stops and starts through the night, so sleep never restores you. NHLBI lists frequent loud snoring, gasping for air and daytime sleepiness and tiredness as signs. 4 The NHS adds being told you make gasping, snorting or choking noises in sleep. 2 No blood test finds it; a clinician confirms it with a sleep study. 4
- 06Iron deficiency and anemia
Anemia, including iron deficiency anemia, and iron deficiency without anemia are both on the MedlinePlus list of common causes of fatigue. 3 Heavy periods are a common cause in women before menopause, and bleeding from the stomach or gut accounts for a share of cases. 12 A clinician decides on iron after a test and looks for the reason the iron is low.
Can be linked with Iron deficiency
- Ferritin
- Low ferritin points to low iron stores, even when hemoglobin is still normal. Under 15 ng/mL fits iron deficiency, and a 30 ng/mL cutoff raises how often it is caught from 25% to 92%; inflammation can push ferritin up and hide a deficiency. 12
- Hemoglobin
- Low hemoglobin means anemia; ferritin and the rest of the blood count help show whether low iron is the reason. See the CBC explainer and anemia.
- 07Recovering from an infection, long COVID or ME/CFS
Illnesses such as glandular fever (mono) and COVID-19 can leave tiredness that lasts weeks. 2 The CDC lists “Tiredness or fatigue that interferes with daily life” and symptoms that get worse after physical or mental effort among the signs of long COVID, and notes there is no laboratory test for it. 15 Exhaustion that crashes for a day or more after effort can point to ME/CFS (myalgic encephalomyelitis, or chronic fatigue syndrome), where pushing through with exercise can make things worse. 1
- 08Underactive thyroid
Hypothyroidism (an underactive thyroid) commonly brings fatigue, weight gain and trouble tolerating cold. 16 Nearly 5 out of 100 Americans ages 12 and older have it, most mildly, and women are much more likely to. 16 It can't be diagnosed from symptoms alone. 16
Can be linked with Hypothyroidism
- 09Prediabetes and diabetes
Diabetes is on the MedlinePlus list of illnesses that cause fatigue. 3 Symptoms of type 2 diabetes can develop slowly, over several years, and many people have none, so tiredness with thirst or passing more urine is worth a blood sugar check. 19 See blood sugar explained.
Can be linked with Prediabetes
- HbA1c
- HbA1c reads average blood sugar over the past 2 or 3 months: below 5.7% is normal, 5.7% to 6.4% is prediabetes and 6.5% or above is diabetes. 20
- Fasting glucose
- After an overnight fast, 99 mg/dL or below is normal, 100 to 125 mg/dL is prediabetes and 126 mg/dL or above is diabetes. 20
- 10Low vitamin B12
People low in B12 are often “easily fatigued with exertion,” and may also notice numbness or tingling. 8 The risk is higher with metformin for more than four months, acid-reducing medicines for more than a year, a vegan or strict vegetarian diet and age over 75. 8 See tingling hands and feet if that is part of it.
Can be linked with B12 deficiency
- Vitamin B12
- A B12 under 150 pg/mL is diagnostic of deficiency; a normal or low-normal result with symptoms leads to a methylmalonic acid test. 8
- Methylmalonic acid
- A high methylmalonic acid (MMA) points to B12 deficiency at the tissue level and is the next step when B12 is borderline. 8
These are things a symptom like this can be linked with, not a diagnosis. Several can sit together, and a clinician who knows your history puts them in order for you.
Cause finder: match the clue to the likely cause and the test
Clues about when the tiredness comes and what travels with it point to a cause faster than any single test. Use the table to see which cause and which test to raise with a clinician. It is a guide to the conversation, not a diagnosis, and several causes often overlap.
| What it feels like, or when it happens | Likely cause to ask about | What checks it |
|---|---|---|
| Dozing off in the day, loud snoring, gasping or choking in sleep | Sleep apnea | A sleep study; no blood test finds it 4 |
| Short or late nights, shift work, an irregular bedtime | Too little sleep | A two-week sleep diary 2 |
| Heavy periods, breathless on stairs, restless legs | Iron deficiency or anemia | Ferritin and a blood count 112 |
| Feeling cold, weight gain | Underactive thyroid | TSH, then free T4 1716 |
| Very thirsty, peeing more, blurred vision | Prediabetes or diabetes | HbA1c or fasting glucose 2019 |
| Numb or tingling hands and feet; metformin, acid reducers or a vegan diet | Low vitamin B12 | Vitamin B12, then methylmalonic acid if borderline 8 |
| Low mood, trouble concentrating, sleeping too much or too little | Depression or anxiety | A conversation with a clinician; no blood test 5 |
| Crashing for a day or more after effort, often after an infection | Long COVID or ME/CFS | No lab test; a clinician's assessment 115 |
| Started soon after a new medicine or a dose change | A medicine side effect | A medicine review with the prescriber 3 |
| Men: low sex drive, low mood, less endurance | Low testosterone | Two morning testosterone tests 21 |
Other causes a clinician may look for
Some causes sit outside the top ten for adults in general but rise to the top for one group. Pregnancy is the obvious one: the NHS lists the hormone changes of pregnancy and menopause among the causes of tiredness. 2 In men, low testosterone can bring low sex drive, low mood, trouble concentrating and less endurance; levels below 300 ng/dL are considered low, and the test usually needs two morning samples. 21 See low testosterone and low libido. Kidney disease, liver disease, heart failure, autoimmune diseases and, rarely, cancer are on the MedlinePlus list as well, usually with other signs alongside. 3 A basic metabolic panel and liver tests check the first two; see kidney function explained and liver enzymes explained. 1
Why am I tired when my blood work is normal?
This is one of the commonest searches, and the answer is reassuring once you see the numbers. In a study of young adults with new fatigue, “only 27% were diagnosed with a condition that could explain the fatigue,” and the commonest of those were anemia, low B12, infection, pregnancy and psychiatric diagnoses. 1 Testing without a reason in the history “may only change treatment in 5% of patients.” 1 The big causes (short sleep, sleep apnea, stress, depression and anxiety) are invisible to a blood test. So a normal panel rules out several treatable problems; it does not mean nothing is wrong.
When a low-normal result is worth a second look
A result inside the lab’s reference range is not always the end of the story. Ferritin is the clearest case: a lab may flag only values under about 15 ng/mL, yet using 30 ng/mL as the cutoff raises the share of iron deficiency caught from 25% to 92%, with specificity (how rarely it calls a false alarm) staying at 98%. 12 B12 is similar: when B12 is normal or low-normal but the story fits, a methylmalonic acid test is the next step. 8 See reference ranges explained for why a range describes most people, not the best value for you.
Ferritin: where an example result fallsng/mL
Can low vitamin D make you tired?
Vitamin D is the test people most often ask for, and the evidence linking it to tiredness is weak. It does not appear among the tests in the 2023 American Family Physician fatigue review. 1 The Endocrine Society’s 2024 guideline says “We suggest against routine testing for 25-hydroxyvitamin D levels in any of the populations studied,” because benefits tied to particular levels have not been shown. 22 A clinician may still check it for a reason such as bone thinning, a gut condition that limits absorption or very little sun; see vitamin D deficiency.
Why am I so tired all of a sudden?
Tiredness that arrives over days points to something that changed: a recent infection, a new medicine, a stretch of short sleep, a period of stress, or early pregnancy. 23 Tiredness that builds over months fits slower causes such as an underactive thyroid, iron deficiency or type 2 diabetes, whose symptoms can develop over several years. 161219 Sudden, severe exhaustion with breathlessness, chest pain or fainting is different and needs emergency care. 6
What tests check for the cause of tiredness?
There is no single fatigue test, and a good history comes first. The 2023 American Family Physician review says the workup “should be guided by history, physical examination, and common causes of fatigue,” and lists the blood tests a clinician chooses from. 1
| Test | What it looks for | When it is added |
|---|---|---|
| Complete blood count, ferritin, iron panel | Anemia, low iron stores | Heavy periods, pale skin, breathlessness, restless legs 112 |
| TSH, then free T4 | Underactive or overactive thyroid | Weight change, feeling cold, low mood 117 |
| HbA1c or fasting glucose | Prediabetes, diabetes | Thirst, frequent urination, family history 119 |
| Vitamin B12, folate, methylmalonic acid | B12 or folate deficiency | Older age, metformin, acid reducers, vegan diet 18 |
| Basic metabolic panel, liver tests | Kidney function, salts, liver | Swelling, less urine, yellow skin, alcohol 13 |
| CRP or ESR | Inflammation | Joint pain, fevers, rashes 1 |
| Sleep study | Sleep apnea | Snoring, gasping, daytime sleepiness 4 |
The panel that checks these
If a clinician tests, a blood count with ferritin, TSH and HbA1c usually come first, because together they check the commonest treatable medical causes. 1 Fasting matters only for a fasting glucose; ferritin and iron are best read alongside CRP, since inflammation raises ferritin. 12 Testosterone is drawn in the morning, and B12 is followed by methylmalonic acid when it is borderline. 821
- FerritinIron deficiency and anemia
- HemoglobinIron deficiency and anemia
- TSHUnderactive thyroid
- Free T4Underactive thyroid
- HbA1cPrediabetes and diabetes
- Fasting glucosePrediabetes and diabetes
- Vitamin B12Low vitamin B12
- Methylmalonic acidLow vitamin B12
- eGFRKidney function, from the basic metabolic panel
- ALTLiver health, from the liver tests
- Total testosteroneLow testosterone in men, two morning samples
- hCGPregnancy, when it is possible
LifeFrom doesn't sell blood tests. It reads results from any lab and helps you prepare for your own doctor; it doesn't diagnose.
Getting a useful test
- Keep a two-week note of bedtime, wake time, naps, alcohol, caffeine, exercise and mood; it often shows the cause before any test. 2
- Say which you mean: tired, sleepy or weak. 1
- List every medicine and supplement with its start date, including antihistamines and sleep aids. 3
- Mention high-dose biotin (common in hair and nail products), which can make TSH look falsely low; the American Thyroid Association advises a gap of at least 2 days before thyroid testing, a step to agree with your clinician. 18
- Mention snoring or gasping, heavy periods, changes in your stools and any weight change. 212
- One result is a snapshot; see why trends beat single readings.
Vitamins and supplements for energy: what helps, and when
No vitamin fixes tiredness that comes from short sleep, stress or low mood. The supplements with real evidence for energy work mainly when a test shows you are low: iron when ferritin (your iron store) is low, vitamin B12 when B12 is low, and vitamin D when it is deficient. 789 CoQ10 has limited evidence, creatine early evidence, and most "energy" products run on caffeine and sugar. 101123 The table ranks them by evidence, using the same grades throughout: Strong, Moderate, Limited, Early, Not supported.
| Supplement | Evidence grade for tiredness | What the trials found | Check first |
|---|---|---|---|
| Iron (a clinician decides) | Moderate, only if iron stores are low | In 144 tired women without anemia, fatigue fell 29% on iron versus 13% on placebo over a month, and only women with a ferritin of 50 ng/mL or less improved 7. Too much iron is harmful, so a clinician decides after a test | Ferritin and a blood count 12 |
| Vitamin B12 | Strong if you are low | People low in B12 are often easily tired with effort; it helps energy only when a test shows you are low 8 | Vitamin B12, then methylmalonic acid if borderline 8 |
| Vitamin D | Limited, only if deficient | In 120 tired young adults with vitamin D under 20 ng/mL, fatigue improved in 72% after one large dose versus 50% on placebo 9. The trial only enrolled people who were deficient, and routine testing in healthy adults is not advised 22 | Vitamin D, if a clinician has a reason to check 22 |
| CoQ10 | Limited | 13 trials with 1,126 people: a small-to-moderate drop in fatigue scores, in healthy people and people with an illness alike 10 | No blood test needed; ask a pharmacist about your medicines |
| Creatine | Early for tiredness | In a small sleep-loss study, one large dose improved thinking speed during 21 hours awake 11. Its stronger evidence is for muscle; it also raises creatinine, a kidney number, on purpose (see the creatine guide) | Tell whoever reads your kidney tests |
| Rhodiola | Limited | 60 adults with burnout felt less fatigued after 28 days than on placebo 24, but NCCIH, the US complementary health institute, finds the evidence not reliable enough for any use 25 | Little is known in pregnancy; an interaction with losartan has been reported 25 |
| Energy drinks and shots | Short-term alertness only | The lift comes from caffeine (70 to 240 mg in a 16-ounce drink) and often 54 to 62 grams of added sugar; they can disturb sleep, and large amounts of caffeine can upset heart rhythm 23 | Not a fix for ongoing tiredness |
| "Adrenal fatigue" supplements | Not supported | Adrenal fatigue is not a recognized diagnosis, and lab tests of 12 such products found unlabelled thyroid hormone in all of them 2627 | See morning cortisol for what a cortisol test can show |
What may help
Most people feel better when the cause is found. The everyday steps below help whatever the cause; the supplements help only in the situations named, and none replaces checking the red flags above.
Start with the everyday things- Keep the same sleep and wake times, and aim for 6 to 9 hours. 2
- Wind down for a couple of hours before bed, with a dark, cool, calm bedroom and no screens in the last hour. 2
- Avoid caffeine, heavy drinking, big meals and hard exercise in the hours before bed. 2
- Move most days if you can; regular exercise helps tiredness linked with many conditions, but if effort leaves you crashed for a day or more, raise it with a clinician before doing more. 1
- If stress or low mood is part of it, talking therapies help; the American Family Physician review recommends cognitive behavioral therapy for fatigue linked with several long-term conditions, though not ME/CFS. 1
- Vitamin B12What people take it for: supporting normal B12 levels, which matters more on a vegan diet and with age. It only helps energy if you are low, which a test shows; people on metformin or acid reducers can ask their clinician about a B12 test. 8
- Vitamin DWhat people take it for: bone and muscle health when sun and diet are short. In one trial it eased tiredness in people who were deficient; that trial did not test people with normal levels. 9
- CoQ10What people take it for: everyday energy. Pooled trials show a small-to-moderate drop in fatigue scores, so expect a modest effect at best. 10
- CreatineWhat people take it for: muscle strength and power. Evidence that it eases tiredness is early, from small sleep-loss studies, and it raises the kidney marker creatinine on purpose. 11
Iron is left off this list on purpose: too much iron is harmful, and low iron needs its cause found, so a clinician decides on iron after a test. Supplements support general wellness. They don't treat the causes above, and if a test finds one, treating that comes first. If you take medicines, are pregnant or have a health condition, check with your clinician before starting anything.
Questions to ask your clinician
- Could any of my medicines be making me tired?
- Should I have a sleep study for snoring or sleep apnea?
- Do my ferritin, B12 or TSH sit low-normal, and is a follow-up test worth it?
- Could this be depression, anxiety or burnout, and what support is there?
- If my tests are normal, what would make you want to look again?
This page is general education, not medical advice or a diagnosis. Tiredness has many causes that can overlap, and the clinician who knows your history and examines you is the one to read your results and put them in order. If tiredness comes with feeling cold or foggy-headed, see always cold and brain fog.
Citations
- Latimer KM, Gunther A, Kopec M. Fatigue in adults: evaluation and management. Am Fam Physician. 2023;108(1):58-69.
- NHS. Tiredness and fatigue.
- MedlinePlus Medical Encyclopedia. Fatigue. US National Library of Medicine.
- National Heart, Lung, and Blood Institute (NHLBI). Sleep apnea: symptoms.
- MedlinePlus Medical Encyclopedia. Major depression. US National Library of Medicine.
- Cleveland Clinic. Fatigue.
- Verdon F, Burnand B, Stubi CL, et al. Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial. BMJ. 2003;326(7399):1124.
- Langan RC, Goodbred AJ. Vitamin B12 deficiency: recognition and management. Am Fam Physician. 2017;96(6):384-389.
- Nowak A, Boesch L, Andres E, et al. Effect of vitamin D3 on self-perceived fatigue: a double-blind randomized placebo-controlled trial. Medicine (Baltimore). 2016;95(52):e5353.
- Tsai IC, Hsu CW, Chang CH, Tseng PT, Chang KV. Effectiveness of coenzyme Q10 supplementation for reducing fatigue: a systematic review and meta-analysis of randomized controlled trials. Front Pharmacol. 2022;13:883251.
- Gordji-Nejad A, Matusch A, Kleedorfer S, et al. Single dose creatine improves cognitive performance and induces changes in cerebral high energy phosphates during sleep deprivation. Sci Rep. 2024;14.
- Short MW, Domagalski JE. Iron deficiency anemia: evaluation and management. Am Fam Physician. 2013;87(2):98-104.
- MedlinePlus Medical Encyclopedia. Gastrointestinal bleeding. US National Library of Medicine.
- Centers for Disease Control and Prevention. Signs and symptoms of stroke.
- Centers for Disease Control and Prevention. Long COVID signs and symptoms.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Hypothyroidism (underactive thyroid).
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Thyroid tests.
- American Thyroid Association. Clinical Thyroidology for the Public: biotin and thyroid function tests. December 2018;11(12):3-4.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Symptoms and causes of diabetes.
- Centers for Disease Control and Prevention. Diabetes testing.
- Cleveland Clinic. Low testosterone (male hypogonadism).
- Endocrine Society. Endocrine Society recommends healthy adults take the recommended daily allowance of vitamin D (guideline news release, June 2024).
- National Center for Complementary and Integrative Health (NCCIH). Energy drinks.
- Olsson EM, von Scheele B, Panossian AG. A randomised, double-blind, placebo-controlled, parallel-group study of the standardised extract SHR-5 of the roots of Rhodiola rosea in the treatment of subjects with stress-related fatigue. Planta Med. 2009;75(2):105-112.
- National Center for Complementary and Integrative Health (NCCIH). Rhodiola: usefulness and safety.
- Cadegiani FA, Kater CE. Adrenal fatigue does not exist: a systematic review. BMC Endocr Disord. 2016;16(1):48.
- Akturk HK, Chindris AM, Hines JM, Singh RJ, Bernet VJ. Over-the-counter "adrenal support" supplements contain thyroid and steroid-based adrenal hormones. Mayo Clin Proc. 2018;93(3):284-290.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
