GPs regularly see patients with the complaint of feeling "tired all the time". It's so common that the acronym TATT is used as shorthand. But what levels of tiredness are normal and when should we seek help for fatigue and exhaustion? Many illnesses like anaemia, diabetes, cancer, infections or depression can all cause symptoms of tiredness and fatigue. Growth spurts, pregnancy and sleep deprivation can too but what about when there's no obvious underlying illness and symptoms are of chronic fatigue? Vivienne Parry investigates.