There is no hard and fast medical definition of tiredness, but most people would agree that it is characterised by a decrease in or absence of energy, as well as physical and mental exhaustion, even when a person is sleeping well. It is important to remember that perceptions and experiences of tiredness are subjective. In other words, what one person considers exhaustion may well be what someone else regards as part of everyday life.

There are some groups of people who seem more likely to experience tiredness than others, such as women and people with long-term physical and mental health conditions. No age group is exempt, although tiredness appears to be less common among the very young and the very old.

The prognosis for tiredness is positive and many people who seek medical advice do not require any follow-up care. A good recovery is especially likely in those whose tiredness is of short duration or low severity. People who have no carer responsibilities, good social support, are not in pain and have good mental health are also more likely to get better without any problems. Men also tend to recover quicker and more completely than women.

On the flip side, the complications of energy levels not being restored include a negative impact on relationships, friendships and work, as well as an increased risk of serious conditions, including obesity, heart disease and diabetes.