Sleep is recognizable by its contrast to wakefulness.

It is a state of reduced awareness and responsiveness, both to internal and external stimuli. This reduced awareness is, however, selective [1]. It is an active process in which the significance of stimuli to the individual is interpreted and this determines whether arousal from sleep occurs. The crying of a child is, for instance, more likely to wake the parent than a different noise of the same intensity.

A second feature of sleep is motor inhibition (Table 1.1). The sleeping subject appears quiescent, but some movements occur, such as rapid eye movements. For each species there is a characteristic posture or type of movement that is adopted during sleep.

Humans usually lie down, but many birds perch while asleep, horses may stand, vampire bats sleep upside down, dolphins and whales swim, and albatrosses can fly. In some of these animals, in contrast to humans, sleep is not accompanied by physical inactivity.

Humans usually sleep with the eyes closed, but some animals, such as cattle, sleep with their eyes open.

Sleep is a cyclical or episodic phase which alternates with wakefulness. There is a wide variation in the duration of sleep between species, but humans, moles and pigs sleep for about 8 out of each 24 h. An important characteristic of sleep which differentiates it from most other states of altered consciousness is that it is promptly reversible.

This is usually recognized by the sleeper, but brief episodes of sleep or wakefulness (microsleeps and microarousals) may emerge from the background state without any subsequent recall of the events.

The margin between sleep and wakefulness is seldom sharp. The transition between wakefulness and sleep often lasts several minutes, and the moment of falling asleep may be impossible to determine.

The point at which sleep is attained, as judged by behavioural criteria, may differ from, for instance, the moment of sleep onset defined by electrophysiological standards. A period of drowsiness is often a transitional phase between wakefulness and sleep, but does not necessarily lead into sleep and may simply be a prolonged episode of subalertness followed by recovery of wakefulness. In a similar way the process of awakening can be sudden, particularly if there is a strong sensory stimulus; or gradual with a stage of partial recovery of wakefulness. Aspects of both sleep and wakefulness coexist in these transitional states, and awareness of the thoughts and images of dreams often persists into wakefulness after apparent arousal from sleep.