Feeling tired all the time is very common, and most of the time the cause is ordinary. Not enough sleep, poor sleep, stress, an unhealthy diet, little exercise and a recent virus top the list. So do hormonal changes such as pregnancy or the menopause, and some medicines. The NHS says it may point to a medical condition if it is unexplained, ongoing and not improving.
The medical causes a GP thinks of first are usually treatable. They include iron deficiency anaemia, an underactive or overactive thyroid, sleep apnoea, depression and diabetes. Most are found with a conversation, an examination and a few blood tests.
This page is about physical tiredness and low energy. If your main problem is thinking clearly, our guide to what causes brain fog is the better starting point.
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Call 112 or 999 in Ireland, 999 in the UK or 911 in the US if:
Get an urgent GP appointment the same day (or NHS 111 in the UK) if:
See a GP soon if:
If you are struggling with your mood or having thoughts of suicide: in Ireland, call Samaritans free on 116 123, Pieta on 1800 247 247, or text 50808 (Text About It). In the UK, call Samaritans on 116 123 or, in England, NHS 111 and choose the mental health option. In the US, call or text 988.
The difference helps your GP. Sleepiness means struggling to stay awake, and points first towards sleep. Fatigue is an overwhelming tiredness not relieved by rest and sleep. The NHS says this kind may have an underlying medical cause. Low energy with low mood points towards mood. Many people have a mix.
On average, adults need 7 to 9 hours of sleep, according to the NHS and HSE. Insomnia, shift work, jet lag and young children all chip away at it.
The NHS says the effects of caffeine can last up to 7 hours. It can disrupt sleep and lead to daytime tiredness. Alcohol may help you fall asleep, but you sleep less deeply afterwards. You can feel tired the next day even after a full night.
The NHS also says regular exercise makes you less tired in the long run, and that stress uses up a lot of energy.
The NHS says glandular fever can cause tiredness lasting several months. COVID-19 symptoms lasting more than 12 weeks are called long COVID, and extreme tiredness is among the most common.
The NHS lists sleepiness as a side effect of drowsy antihistamines, such as chlorphenamine, diphenhydramine and hydroxyzine. It lists tiredness as a side effect of beta blockers. If tiredness started with a new medicine, tell your prescriber. Do not stop a prescribed medicine on your own.
These are the conditions the NHS and HSE link most often to tiredness. The clues below are what tends to set each one apart. They are not a way to diagnose yourself.
A few of these deserve more detail.
B12. The NHS says B12 or folate deficiency affects around 1 in 10 people aged 75 or over in the UK. Some nerve problems can be permanent if it is not treated.
Thyroid. An underactive thyroid usually develops slowly, with mild symptoms. Our thyroid guide covers the test.
Diabetes. Many people with type 2 diabetes have no symptoms. Type 1 symptoms can get worse quickly, which is why it is in the urgent box.
Sleep apnoea. The NHS suggests bringing anyone who has seen your night-time symptoms to the appointment. The STOP-BANG questions are a quick screen.
Depression. The NHS advises seeing a GP if you have symptoms most of the day, every day, for more than 2 weeks. It is treatable, and what works, ranked by evidence is covered separately.
Long COVID and ME/CFS. In ME/CFS, symptoms get worse after physical or mental activity, sometimes for weeks. This post-exertional malaise changes what advice is safe, as our guide explains.
Most tiredness is not a sign of anything serious. But the NHS says to see a GP as soon as possible if you are losing weight without trying and have other symptoms. Night sweats with unexplained weight loss is another reason to book soon. In very rare cases, a severely underactive thyroid becomes an emergency, more often in older people.
Your GP will ask about your symptoms, lifestyle and medical history. They may ask about family thyroid problems or a recent pregnancy, and examine your neck.
It helps to arrive with notes on:
A sleep diary for a week or two helps. The HSE suggests noting bed and wake times, time to fall asleep, night waking and drinks.
The NHS says a GP may arrange blood tests for anaemia, diabetes or an overactive thyroid. NICE's Clinical Knowledge Summary for GPs in England sets out a first-line list. An audit by researchers at Ninewells Hospital in Dundee reported the tests on that list. These include:
Not everyone needs every test, and practice varies between Ireland, the UK and the US. The same audit reviewed 16,889 GP blood test requests from 2019 that mentioned tiredness. A full blood count was requested in 89%. Ferritin was requested in only 9.4%. If iron is a possibility for you, it is reasonable to ask whether ferritin is included. Our guide to the blood tests worth asking about explains how to prepare, including which supplements can distort results.
A result outside the laboratory range is not a diagnosis on its own. Ask your GP what each result means for you, and ask for the actual numbers.
That is useful, not a dead end. The NHS says treatment depends on the cause. It might be advice on sleep or lifestyle, or a talking therapy such as CBT. If the cause is unclear, your GP may refer you to a specialist. Sleep apnoea and depression do not show up on blood tests, so make sure they have been asked about.
These suggestions come from NHS and HSE guidance. They do not replace seeing a GP.
No supplement treats tiredness. If a test shows a deficiency, your GP will advise you on how it should be treated.
Time in bed is not the same as good sleep. Sleep apnoea disturbs your breathing at night and leaves you tired by day. Depression, anxiety, an underactive thyroid and anaemia can all cause tiredness despite enough sleep. If you wake unrefreshed most days, see your GP.
Your GP will decide based on your symptoms. The list above is a typical first round. If iron deficiency is possible, ask whether ferritin is included.
Yes. The NHS says stress uses up a lot of energy, and getting tired easily is a symptom of generalised anxiety disorder. But do not assume stress is the whole story until other causes are considered.
Most is not. Seek urgent help if it comes with severe breathlessness, chest pain, extreme thirst, vomiting or confusion. See a GP soon if it comes with weight loss, night sweats or mood changes, or lasts weeks unexplained.
This article is general information, not medical advice. It cannot diagnose the cause of your tiredness. Talk to your GP, pharmacist or prescriber about your own symptoms and before changing any medicine.
Reviewed by: Proco Team
Last updated: November 2026