Insomnia is a subjective complaint encompassing delay in onset, insufficient duration and poor quality of sleep. Not all three symptoms need to be present for the diagnosis of insomnia to be made. Importantly, most sufferers feel tired or distressed during the day.

The primary sleep disturbance is not associated with another medical disorder such as major depression and not due to the effects of a substance, for example caffeine, or a general medical condition, for example arthritis leading to pain. Insomnia is an extremely common condition with an overall prevalence of 27% (range 8% to 43%) among primary care attendees. The prevalence of sleep problems rises from about 15% in 20-54 year olds, to 30% in the over-55 year olds. Women are 1.5 times more likely than men to have this sleep disorder. Insomnia is associated with negative consequences for health-related quality of life, daytime well-being and has deleterious economic implications.