People suffer from insomnia for a number of reasons, which can be divided into the following different categories:

Psychological

Psychological causes of insomnia include crisis, worry, relationship difficulties, bereavement and stress. Concerns about not being able to sleep, in particular, can have a self-perpetuating effect on insomnia, which can carry on even when the original problem has been resolved. Insomnia is also commonly associated with other health problems, such as depression, anxiety and dementia.

Environmental

Disruptions within the sleeping environment or to bedtime routines can cause insomnia. Key culprits include noise; excessive light, heat or cold; an unfamiliar bed; increased physical or intellectual activity before bedtime (e.g. exercise or reading), and a partner's snoring or fidgeting.

Physical

Insomnia can also be triggered by underlying physical conditions that cause involuntary movement, pain or discomfort. These conditions include:

• Movement disorders, e.g. restless leg syndrome
• Respiratory problems, e.g. coughing
• Painful conditions, e.g. headaches, arthritis and back pain
• Hormonal causes, e.g. hot flushes due to the menopause
• Other physical illness, e.g. pruritus (excessive itching), Parkinson's disease and gastrointestinal disorders (e.g. ulcers).

Medicines

Sleeping problems can also result from the effects of medicines. Stopping certain drugs, such as antidepressants or hypnotics, can cause 'rebound' insomnia. In addition, other medicines can produce side effects that reduce the quality of sleep. Examples include appetite suppressants; certain antidepressants; thyroid hormones; corticosteroids (which can cause agitation); beta blockers (which may result in bad dreams), and diuretics (which may increase bed wetting).

Many illicit recreational drugs, such as amphetamines, MDMA (ecstasy) and cocaine, have stimulant effects that contribute to insomnia.

Alcohol can help people get to sleep, but is associated with early morning wakefulness. Stimulants such as caffeine (found in tea, coffee, chocolate and some painkillers) and nicotine can also cause sleeping difficulties.

Disruption of the sleep cycle

This is a type of insomnia where a person's biological rhythm of sleep and wakefulness (known as the circadian rhythm) is out of phase with the conventional sleep-wake pattern of sleeping at dusk and rising at dawn. It is typically short-lived and the most common causes are jet lag and shift work.