1. Coeliac disease
This is a type of food intolerance, where your body reacts badly when you eat gluten – a substance found in bread, cakes and cereals.
One in 100 people in the UK are affected, but research suggests up to 90% of them don't know they have the condition, according to patient group Coeliac UK.
Other symptoms of coeliac disease, apart from tiredness, are diarrhoea, anaemia and weight loss. Your GP can check if you have coeliac disease through a blood test.
Read more about coeliac disease.
One of the most common medical reasons for feeling constantly run down is iron deficiency anaemia. It affects around 1 in 20 men and postmenopausal women, but may be even more common in women who are still having periods.
Typically, you'll feel you can't be bothered to do anything, your muscles will feel heavy, and you'll get tired very quickly.
Women with heavy periods and pregnant women are especially prone to anaemia.
Read more about iron deficiency anaemia.
3. Chronic fatigue syndrome
Chronic fatigue syndrome (myalgic encephalomyelitis, or ME) is a severe and disabling tiredness that goes on for at least six months. There are usually other symptoms, such as a sore throat, muscle or joint pain, and headache.
Read more about chronic fatigue syndrome.
4. Sleep apnoea
Sleep apnoea is a condition where your throat narrows or closes during sleep and repeatedly interrupts your breathing.
This results in bad snoring and a drop in your blood's oxygen levels. The difficulty in breathing means you wake up often in the night and feel exhausted the next day.
It's most common in overweight middle-aged men. Drinking alcohol and smoking makes it worse.
Read more about sleep apnoea.
5. Underactive thyroid
An underactive thyroid gland means you have too little thyroid hormone (thyroxine) in your body. This makes you feel tired.
You're also likely to put on weight and have aching muscles. It's most common in women and happens more often as you get older.
Your GP can diagnose an underactive thyroid by taking a blood test.
Read more about having an underactive thyroid.
One of the main symptoms of diabetes, a long-term condition caused by too much sugar in the blood, is feeling very tired. The other key symptoms are feeling very thirsty, going to the toilet a lot, and weight loss. Your GP can diagnose diabetes with a blood test.
Read more about diabetes and find out how to make smart sugar swaps.
Find your local diabetes support services.
7. Glandular fever
Glandular fever is a common viral infection that causes fatigue, along with fever, sore throat and swollen glands.
Most cases happen in teenagers and young adults. Symptoms usually clear up within four to six weeks, but the fatigue can linger for several more months.
Read more about glandular fever.
As well as making you feel very sad, depression can also make you feel drained of energy. It can stop you falling asleep or cause you to wake up early in the morning, which makes you feel more tired during the day.
Read more about depression.
Find your local depression support services and your local depression self-help groups.
9. Restless legs
This is when you get uncomfortable sensations in your legs, which keep you awake at night.
You might have an overwhelming urge to keep moving your legs, a deep ache in your legs, or your legs might jerk spontaneously through the night.
Whatever your symptoms, your sleep will be disrupted and of poor quality, so you'll feel very tired throughout the day.
Read more about restless legs.
Feeling anxious is sometimes perfectly normal. But some people have constant uncontrollable feelings of anxiety, which are so strong they affect their daily life.
Doctors call this generalised anxiety disorder (GAD). It affects around 1 in 20 people in the UK. As well as feeling worried and irritable, people with GAD often feel tired.
Read more about anxiety.
Find your local anxiety support services.