Tiredness ranks among the most frequent complaints family doctors hear, and a hidden serious disease is rarely what lies behind it. In a 2016 systematic review of 26 primary-care studies, about 1 in 5 people who raised tiredness with a family doctor had depression (18.5 percent), while serious physical disease, anemia and cancer included, was found in about 4 in 100 (4.3 percent).¹

The usual reasons behind “why am I always tired?” are ordinary ones, the UK’s NHS says: too little sleep or poor sleep, stress, an unhealthy diet or little exercise, and demanding life events.² Tiredness that lasts for weeks with no clear reason can also come from a medical condition such as anemia, a thyroid problem, diabetes, sleep apnea or depression, which only a doctor can diagnose.²³

Sleep, stress and daily life: the everyday answers to “why am I always tired”

The UK’s NHS lists the common causes of tiredness as not getting enough sleep or having insomnia, an unhealthy diet or little exercise, stress, depression and life challenges such as bereavement or a new baby, hormonal changes such as pregnancy or the menopause, some illnesses such as glandular fever and COVID-19, and some medical treatments.²

MedlinePlus, the US National Library of Medicine’s health site, says fatigue can be a normal and important response to physical activity, emotional stress, boredom or lack of sleep, and is usually not due to a serious disease. It adds that fatigue which enough sleep, good nutrition and a low-stress environment do not relieve should be checked by a health care provider.³

Some causes, such as a new baby, shift work or grief, cannot be changed quickly, and the advice below may fit only partly. If short or broken nights are part of the picture, start with the sleep habits that have the best evidence; if pressure at work or home is, read what stress does in the body.

Depression is found more often than serious physical disease

Depression is the diagnosis doctors find most often in people who come to them tired, and serious physical disease is uncommon, according to a 2016 systematic review by Rebekka Stadje and colleagues at the University of Marburg in Germany. The review drew on primary-care studies from Europe, North America, Australia and Japan.¹

The study

Moderate evidence

Stadje and colleagues' 2016 review of tiredness in primary care

The Marburg team looked for studies in which patients brought tiredness to a family doctor as the main or a secondary complaint. Pooling only studies with clear diagnostic criteria and suitable testing, they found depression in 18.5 percent of patients, anemia in 2.8 percent, cancer in 0.6 percent and serious physical disease of any kind in 4.3 percent. In the studies that compared tired patients with patients who were not tired, physical disease was about as common in both groups, while depression was more common among the tired.¹

Because physical disease was as common in people who were not tired, the authors suggest it may sometimes accompany tiredness rather than cause it, and conclude that extensive testing is warranted only when the history or examination points to something specific. The main caveat is age: the search ran in 2010.¹ A much larger 2024 study of English GP records partly disagrees: among about 305,000 people who saw a GP with new tiredness in 2007 to 2017, more than half of 237 diseases studied, anemia included, were diagnosed more often within a year than in people who were not tired; depression was again among the most strongly linked.⁵

A Dutch study that followed patients for a year points the same way. Of 571 adults who saw a GP with a new episode of fatigue as their main complaint, 46.9 percent received at least one diagnosis in the next year that could be linked to it, mostly symptom labels such as musculoskeletal or psychological problems, and 8.2 percent were diagnosed with clear physical disease, including anemia, thyroid problems, diabetes and cancer. The authors wrote that half got no diagnosis that could explain their tiredness. It was one observational study, recruited in 2004 to 2006, about three-quarters of them women.⁶

A 2021 review in the German journal Deutsches Ärzteblatt, co-written by two of the 2016 review’s authors, still uses those figures. It names sleep disorders, breathing problems during sleep, depression and heavy psychosocial stress as the most common causes of persistent fatigue, and says fatigue is a main or secondary reason for 10 to 20 percent of visits to a primary-care doctor.⁴

Medical causes of constant tiredness worth raising with a doctor

Health services in the US and UK name the same medical causes of tiredness: depression, anemia, thyroid problems, diabetes, sleep apnea, some medicines, infections such as COVID-19, and ME/CFS.²³ The NHS advises against self-diagnosing from symptoms and suggests seeing a GP if you are worried.²

The table lists other signs from the health services’ symptom pages; having some does not mean you have the condition.

Depression is a medical diagnosis with set criteria. The US National Institute of Mental Health (NIMH) lists fatigue, lack of energy or feeling slowed down among its symptoms, and says a diagnosis needs symptoms most of the day, nearly every day, for at least 2 weeks, one of them a depressed mood or loss of interest. NIMH also notes that some medicines and conditions, such as viruses or thyroid disorders, can mimic depression, which a provider can rule out with an examination, an interview and lab tests.⁷

An underactive thyroid develops slowly: the NHS says its symptoms may be mild and tend to get worse over time, and it is checked with blood tests.⁹ The US National Institute of Diabetes and Digestive and Kidney Diseases cautions that many of its symptoms, especially fatigue and weight gain, are common and do not necessarily mean a thyroid problem; women and people over 60 are more likely to have it.¹⁴

Sleep apnea is easy to miss because its main signs happen while you are asleep. The NHS suggests asking someone to watch you sleep, and lists daytime tiredness, trouble concentrating, mood swings and headaches on waking as signs.¹¹ The US National Heart, Lung, and Blood Institute (NHLBI) says the daytime sleepiness and tiredness it brings can lead to problems with learning, focusing and reacting.¹⁵

Some medicines can cause tiredness or drowsiness too: MedlinePlus names antihistamines for allergies, blood pressure medicines, sleeping pills, steroids and diuretics (water pills), and says some antidepressants can cause or worsen fatigue.³

ME/CFS is defined differently on each side of the Atlantic. MedlinePlus describes chronic fatigue syndrome as fatigue lasting at least 6 months that rest does not resolve, diagnosed after other causes are ruled out.³ The UK’s National Institute for Health and Care Excellence (NICE), in its 2021 guideline, says to suspect ME/CFS after 6 weeks of four core symptoms in adults, and to diagnose it once they have lasted 3 months without another explanation.¹³

Myth

If you're always tired, you probably need an iron or vitamin supplement.

Fact

Anemia turned up in fewer than 3 in 100 tired primary-care patients in a 2016 review. The NHS recommends iron tablets when a blood test shows a low red blood cell count, and says to keep iron tablets out of children's reach, because an overdose can be fatal to a young child.

Before recommending iron, the NHS says, a GP looks for the reason for the anemia, such as heavy periods or an ulcer.⁸ MedlinePlus suggests discussing with your provider whether any vitamins might help.³

What a doctor will usually ask and check

A doctor seeing you about tiredness will usually start with questions and an examination, according to MedlinePlus: your medical history, the tiredness itself, your lifestyle, habits and feelings, then a check of the heart, lymph nodes, thyroid, abdomen and nervous system.³

Tests may include blood tests for anemia, diabetes, inflammation and infection, kidney, liver and thyroid tests, and a urine test, MedlinePlus says.³ The NHS mentions blood tests for anemia, diabetes or an overactive thyroid, and says a GP may refer you to a specialist if the cause stays unclear.²

1. 1Your accounthistory, sleep, mood, medicines, life events
2. 2Examinationheart, glands, thyroid, abdomen, nerves
3. 3Basic testsblood and urine, if the history suggests them
4. 4More testsonly when something points to a cause
5. 5Follow-upregular reviews if the cause stays unclear

German guidance limits tests further. The 2021 review, summarizing the 2018 fatigue guideline of the German College of General Practitioners and Family Physicians (since updated in 2023), recommends a short set of basic blood tests, more only when something points to a specific condition, and, if the cause stays unclear, watchful waiting with follow-up every 4 to 6 weeks to avoid overdiagnosis.⁴

Treatment depends on the cause. The NHS lists talking therapies such as cognitive behavioral therapy (CBT), sleep advice and help with relaxation or lifestyle among the options a GP may offer.²

Habits to try while you arrange an appointment

Regular sleep times, less alcohol, no smoking, regular exercise and a balanced diet are the everyday steps the NHS and MedlinePlus suggest for tiredness.²³ They are worth trying while you wait, but are no reason to delay if you have any warning signs listed in the next section.

1. Give sleep a regular slot

On its tiredness page, the UK’s NHS suggests sticking to the same sleep times and aiming for 6 to 9 hours of sleep, winding down for a couple of hours before bed, keeping the bedroom not too bright, noisy or warm, and avoiding screens in the last hour.² The NHS insomnia page gives 7 to 9 hours as the average adult’s need and advises no tea or coffee for at least 6 hours before bed.¹⁶

2. Keep alcohol, nicotine and sedatives in check

The NHS advises not smoking and not drinking too much alcohol.² MedlinePlus recommends avoiding alcohol, nicotine and drug use, and notes that sedatives tend to make fatigue worse.³

3. Move and eat regularly, unless activity makes you worse

Regular exercise and a healthy, balanced diet are on both the NHS and MedlinePlus lists.²³ There is an important exception. If ordinary activity leaves you much worse hours or days later, which NICE calls post-exertional malaise, talk to a doctor before doing more: its 2021 guideline says people with ME/CFS should not be offered general exercise programs or graded exercise therapy.¹³

4. Ask about your medicines, without stopping them

If your tiredness started around the time you began a new medicine, ask a doctor or pharmacist about it. Keep taking it until you have: MedlinePlus warns against stopping or changing any medicine without first talking to your provider.³

5. Write down what you will tell the doctor

Note when the tiredness started, how it affects your day, your sleep and mood, other symptoms, and every medicine and supplement you take. The 2021 German review says a symptom diary can help structure the conversation, and the NHS suggests bringing someone who has seen you sleep if you might have sleep apnea.⁴¹¹

WiserHours

Before you see a doctor about tiredness

• Same sleep and wake times for a couple of weeks
• No tea or coffee in the 6 hours before bed
• Alcohol cut back, no smoking
• Note when the tiredness started and how it affects your day
• List every medicine and supplement you take
• Note other symptoms, such as weight change, thirst, low mood or snoring
• Ask someone whether you snore or gasp in your sleep

Find this checklist online:
https://wiserhours.com/energy/why-am-i-always-tired/#checklist-why-am-i-always-tired-before-appointment

When to see a doctor about tiredness, and how urgently

Thoughts of suicide or self-harm, confusion, or passing little or no urine need help at once. Tiredness that has lasted a few weeks without a clear reason, or that comes with other symptoms, needs a doctor’s appointment. We sorted NHS, MedlinePlus and NIMH advice by urgency.

• Immediately: if you are thinking about suicide or harming yourself, call or text the free, confidential 988 Suicide & Crisis Lifeline in the US, open 24/7, or call 911 if a life is in danger.¹⁷⁷ In the UK, the NHS says to call 999 or go to A&E if a life is at risk or you cannot keep yourself or someone else safe, and to call NHS 111 and choose the mental health option for urgent help; elsewhere, call your local emergency number.¹⁸ MedlinePlus says to contact a provider right away for confusion or dizziness, blurred vision, little or no urine, or recent swelling and weight gain; in the UK, NHS 111 can help if you are not sure whether to call 999.³¹⁹ Do not drive when drowsy, MedlinePlus advises.²⁰ In the UK, the NHS says confirmed sleep apnea with excessive sleepiness means not driving until the tiredness is under control.¹¹
• Book an appointment: the NHS suggests seeing a GP when tiredness has gone on for a few weeks without an obvious reason, gets in the way of daily life, or comes with other symptoms such as weight loss or mood changes, or with gasping, snorting or choking noises in your sleep.² MedlinePlus adds unexplained fatigue with a fever, unintended weight loss or regular sweats; constipation, dry skin, weight gain or feeling unable to tolerate cold; feeling sad or depressed; waking many times in the night, insomnia or frequent headaches; and taking medicines or drugs, prescribed or not, that may cause tiredness.³ NIMH advises talking to a health care provider if signs of depression persist.⁷

The bottom line

Constant tiredness often has everyday causes, and in a 2016 review of people who took it to a family doctor, depression turned up far more often than anemia, cancer or other serious physical disease.²¹ Sort out what you can, sleep, alcohol and medicines included, but if the tiredness has lasted a few weeks without a clear reason, affects your daily life or comes with other symptoms, see a doctor and bring your notes.²