Feeling tired all the time is extremely common, but persistent fatigue should not always be dismissed as simply being busy or not sleeping enough. Around 13 per cent of people in the UK report experiencing persistent fatigue, according to a YouGov survey, and tiredness is one of the most frequent reasons people seek medical advice. Poor sleep, stress, anaemia, thyroid problems, sleep apnoea and conditions such as diabetes can all contribute.

While lifestyle factors are often behind low energy levels, ongoing tiredness can sometimes point to an underlying issue that needs attention. Understanding the possible causes can help identify whether simple changes to daily habits may be enough or whether it is time to speak to a healthcare professional.

1. Poor Sleep Quality Or Not Getting Enough Sleep

Poor sleep is the most common explanation for feeling exhausted. While occasional restless nights are normal, regularly failing to get enough restorative sleep can have a major impact on energy levels, mood and concentration.

The NHS recommends that most adults aim for around seven to nine hours of sleep per night, although sleep quality is just as important as duration.

Common causes of poor sleep include inconsistent bedtimes, excessive screen use before bed, caffeine late in the day, alcohol consumption, anxiety and an uncomfortable sleeping environment.

A 2023 study by the Sleep Foundation found that up to two-thirds of adults worldwide occasionally experience symptoms of insomnia, highlighting how widespread sleep difficulties are.

2. Stress And Burnout

Long-term stress is another major contributor to persistent tiredness. When the body remains under pressure for prolonged periods, elevated stress hormones can affect sleep, immune function and concentration.

Work-related stress is particularly common in the UK. Data from the Health and Safety Executive (HSE) shows that stress, depression or anxiety accounted for around half of all work-related ill health cases in Great Britain in recent years.

Stress-related fatigue may improve through better boundaries, relaxation techniques, exercise and psychological support where needed.

3. Iron Deficiency And Anaemia

Iron deficiency anaemia is one of the most common medical causes of tiredness. Iron is essential for producing haemoglobin, which carries oxygen around the body.

When iron levels are low, people may experience fatigue, dizziness, shortness of breath, headaches and paleness.

Women with heavy periods, pregnant women, and people following diets low in iron are among those at higher risk.

According to the World Health Organisation, anaemia affects approximately 30% of women 15–49 years of age globally, although rates vary significantly between countries.

A GP can diagnose anaemia through a blood test and recommend treatment if needed.

4. Thyroid Problems

An underactive thyroid (hypothyroidism) can slow the body's metabolism and leave people feeling constantly drained.

The condition affects two in every 100 people in the UK, according to The British Journal of General Practice, and is more common among women and older adults.

Other symptoms include weight gain, feeling unusually cold, constipation, dry skin and difficulty concentrating.

A simple blood test can check thyroid hormone levels, and treatment usually involves thyroid hormone replacement medication.

5. Obstructive Sleep Apnoea

Sleep apnoea is a common but frequently overlooked cause of ongoing tiredness. The condition causes repeated interruptions to breathing during sleep, preventing the body from getting enough restorative rest.

Dr Tom Chambers, Co-founder and Chief Medical Officer of Theta Sleep, says fatigue caused by sleep apnoea is often mistaken for normal tiredness.

'Persistent tiredness is one of the most common reasons people see their GP, yet the cause is often missed because we tend to treat exhaustion as a lifestyle problem rather than a medical sign. In sleep medicine, the diagnosis I see overlooked most is obstructive sleep apnoea.'

He adds:

'Patients assume snoring is harmless and that feeling shattered after a full night in bed is just stress or getting older. Often it is not.'

Warning signs include loud snoring, breathing pauses noticed by a partner, waking unrefreshed despite enough sleep and struggling to stay awake during the day.

6. Depression And Low Mood

Fatigue is one of the most common symptoms of depression. It can affect motivation, concentration, sleep quality and the ability to complete everyday tasks.

According to the NHS, around one in four adults in England experiences at least one diagnosable mental health problem in any given year, especially anxiety and depression.

Depression-related tiredness may occur even when someone appears to be functioning normally.

7. Diabetes

Diabetes can cause tiredness because the body is unable to properly use glucose from food for energy.

Type 2 diabetes often develops gradually, and fatigue may appear alongside symptoms such as increased thirst, frequent urination, blurred vision and unexplained weight loss.

Diabetes UK estimates that more than 4.7 million people in the UK are living with diabetes, with around 1.3 million more thought to have undiagnosed type 2 diabetes.

Early diagnosis and management can reduce the risk of complications.

8. Vitamin Deficiencies

Low levels of vitamin B12, vitamin D and other nutrients may contribute to fatigue.

Vitamin B12 deficiency is particularly important because it can also cause neurological symptoms, including numbness, tingling and problems with balance.

The NHS notes that some groups, including older adults, people following vegan diets and those with certain digestive conditions, may be more vulnerable to vitamin B12 deficiency.

9. Long-Term Infections And Inflammatory Conditions

Some illnesses can cause fatigue that continues long after the initial infection has cleared.

For example, glandular fever (infectious mononucleosis) can cause prolonged exhaustion, particularly among teenagers and young adults.

Persistent fatigue can also occur with inflammatory and autoimmune conditions, where the immune system mistakenly attacks healthy tissues.

10. Medication Side Effects

Some medicines can cause tiredness or drowsiness. These include certain antihistamines, antidepressants, sleeping tablets and some blood pressure medications.

If fatigue begins after starting a new medication, speak to a healthcare professional before making changes.

11. Hormonal Changes

Hormonal changes during pregnancy, perimenopause and menopause can affect sleep, mood and energy levels.

The NHS estimates that around 13 million women in the UK are currently menopausal or post-menopausal, meaning millions may experience symptoms linked to changing hormone levels.

Hot flushes, night sweats and disrupted sleep can all contribute to daytime exhaustion.

12. Chronic Fatigue Syndrome (ME/CFS)

ME/CFS is a long-term condition characterised by severe fatigue that is not improved by rest.

NICE guidance highlights three key symptoms: persistent fatigue, post-exertional malaise (symptoms worsening after activity) and unrefreshing sleep.

Research suggests ME/CFS affects around 404,000 people in the UK.

When Should I See My GP?

Book a GP appointment if:

  • Your tiredness lasts several weeks or keeps returning
  • It affects your ability to work or complete daily activities
  • You have unexplained weight loss or weight gain
  • You experience breathlessness, dizziness or heart palpitations
  • You have changes in your mood or sleep pattern
  • Rest and lifestyle changes do not improve your energy levels

Your GP may discuss your symptoms and arrange blood tests to check for conditions such as anaemia, thyroid problems and diabetes.

When Should I Go To A&E?

Seek urgent medical attention if tiredness occurs with:

  • Chest pain
  • Severe breathing difficulties
  • Fainting
  • Sudden confusion
  • Stroke symptoms such as facial drooping, arm weakness or speech problems
  • Severe dehydration

Feeling tired all the time is not something you should simply accept as part of a busy lifestyle or getting older. While poor sleep and stress are common causes, persistent fatigue can sometimes signal an underlying medical condition that needs treatment. If tiredness continues despite healthy lifestyle changes or is affecting your daily life, speaking to your GP can help identify the cause and ensure you receive the appropriate care.