Why Am I Always Tired? Common Causes of Fatigue
Understand why you might be feeling tired all the time, the medical causes to rule out, and practical steps to boost your energy.
Lifestyle Factors — The Usual Suspects
Before considering medical causes, review your daily habits — they account for the majority of fatigue cases. Sleep quality matters more than quantity: going to bed and waking up at inconsistent times disrupts your circadian rhythm. Screen use before bed suppresses melatonin, the hormone that makes you feel sleepy. Physical activity paradoxically increases energy — people who are sedentary and start exercising regularly usually feel less tired within a few weeks. Nutrition plays a huge role: skipping breakfast, eating too many refined carbohydrates (causing blood sugar crashes), and insufficient protein all contribute. Even mild dehydration causes noticeable fatigue. Small, consistent changes often produce dramatic results: try a 7-day experiment of sleeping at the same time, drinking 6 to 8 cups or glasses of fluid a day, and walking 20 minutes daily.
Medical Causes Worth Investigating
If lifestyle changes don't help after 4 weeks, it's time to see your GP. Iron deficiency anaemia is the most common medical cause, especially in women with heavy periods — a simple blood test (full blood count) reveals this. Underactive thyroid (hypothyroidism) is a common condition. The NHS says its usual cause, Hashimoto's disease, is most common in women, though anyone can get an underactive thyroid. It causes fatigue alongside weight gain, dry skin, and feeling cold. It's diagnosed with a TSH blood test and easily treated with daily medication. Vitamin D deficiency is common in the UK because of limited winter sunlight — NHS advice is that everyone should consider a daily 10 microgram supplement during autumn and winter — and it causes fatigue, muscle weakness, and mood changes. Type 2 diabetes causes fatigue through poor blood sugar regulation, alongside thirst and frequent urination. Coeliac disease (gluten intolerance) affects at least 1 in every 100 people in the UK according to the NHS, and is often undiagnosed — symptoms include fatigue, bloating, and anaemia.
Mental Health and Fatigue
Depression, anxiety, and chronic stress are major — and often overlooked — contributors to fatigue. Depression doesn't always present as sadness; 'tired all the time' is one of the most common ways it manifests. Sleep may be disrupted (insomnia or oversleeping), and even adequate sleep feels unrefreshing. Anxiety causes mental exhaustion through constant hypervigilance and worry. Chronic stress keeps cortisol elevated, which disrupts sleep architecture and depletes energy reserves. If emotional symptoms accompany your tiredness — loss of interest in things you enjoy, persistent worry, irritability, or feeling overwhelmed — speak to your GP. Talking therapies (particularly CBT) and lifestyle changes are highly effective. In England the NHS offers free talking therapy through NHS Talking Therapies for anxiety and depression (formerly IAPT). You can refer yourself without seeing a GP first, as long as you are registered with a GP and aged 18 or over — 16 or over in some areas. Scotland, Wales and Northern Ireland have their own services; start with your GP.
When to Get Blood Tests
Ask your GP for blood tests if fatigue lasts more than 4 weeks despite genuine lifestyle improvements. The standard fatigue screen typically includes: Full Blood Count (FBC) for anaemia; Thyroid Function Tests (TSH and T4); Iron studies (ferritin — the storage form of iron, often low even when FBC is normal); Vitamin D level; Blood glucose or HbA1c for diabetes; Kidney and liver function tests; and inflammation markers (CRP/ESR). If these are all normal and fatigue persists for 3+ months with other symptoms (post-exertional malaise, cognitive difficulties, unrefreshing sleep), discuss the possibility of Chronic Fatigue Syndrome (ME/CFS) with your GP. This is a real medical condition, not 'just tiredness', and requires specialist management.
- fatigue
- energy
- blood tests
Sources and further reading
This page was written from the following published guidance. All links checked 26 August 2026.