The many abnormalities of sleep include excessive sleep, inability to sleep, restless sleep, nightmares, bed-wetting, sleep paralysis, restless leg syndrome, snoring and other problems. Neurologists classify sleep disorders under three headings: hypersomnia, insomnia, and nocturnal behavioural symptoms.
It is still not clear why we need to spend almost a third of our lives asleep. It was originally assumed that sleep was needed for physical recuperation. But it now seems more likely that it is primarily for memory, concentration and learning. When people do not get enough sleep, their memory suffers.
The normal sleep cycle involves distinct stages from light drowsiness to deep sleep. REM (rapid eye movement) sleep is a different type of sleep, where the eyes move rapidly and vivid dreaming is most common. During a night, there will be several cycles of non-REM and REM sleep.
Surveys in many cultures show that sleep disorders of some sort bother about one-third of the population, often being associated with workers whose sleep schedules are disturbed by rotating shifts. In Europe, sleep disorders account for about one in seven visits to the doctor. Many sleep disorders affect people in middle age.
Many diseases affect sleep and it is increasingly appreciated how sleep has significant effects on diseases themselves. Neurologic disorders that may be particularly important include epilepsy, stroke, dementia, and chronic pain. A variety of pulmonary and cardiovascular diseases are affected by sleep including asthma, thrombotic disease, chronic obstructive pulmonary disease, and cardiovascular disease.