External influences and some internal stimuli have less influence on the brain during sleep than during wakefulness. To an extent the brain becomes deafferentated, and partially deprived of sensory stimulation. This has in the past led to the concept of sleep as a passive phase, contrasting with the active state of wakefulness. This concept has been discarded in the light of physiological findings over the last 50 years.

Most parts of the brain are active in sleep, although their functions and interrelationships differ from wakefulness.

Until the 1950s it was assumed that sleep was a homogeneous or unitary phenomenon which was the opposite of wakefulness. Electrophysiological studies in the 1950s, however, clearly demonstrated that there were two main states of sleep, non-rapid eye movement (NREM) and rapid eye movement (REM) sleep (Table 1.2). The fact that they usually occurred in sequence without an intervening episode of wakefulness probably delayed their recognition.

Both NREM and REM sleep are themselves heterogeneous and at any one moment these two states and wakefulness may not be as distinct as has previously been thought. There is probably a continuous tendency to move in or out of one of these three states throughout the day and night. At any time, parts of the brain may be predominantly in, for instance, REM sleep whereas another part may be tending towards NREM sleep.

The current methods of categorizing sleep into one or other state by conventional electrophysiological criteria give a false sense of rigidity to the constantly changing functional processes within the brain.

The exact onset of sleep is difficult to identify since precise criteria based on changes in subjective awareness, behavioural features or electrophysiological changes are hard to establish _[2]. The lighter stages of NREM sleep appear first, and often alternate with brief episodes of wakefulness before the deeper NREM sleep stages are entered. NREM sleep, and particularly its deeper stages, predominates early in the night, but REM sleep appears at around 90-min intervals. There are usually four to six of these sleep cycles each night and as the night progresses the REM episodes become longer, and NREM sleep both shorter and lighter. Brief arousals to wakefulness are a normal feature of sleep.

The drive to enter sleep increases with the duration since the last sleep episode (homeostatic drive). This is integrated with the circadian drive to sleep and wakefulness, which varies over each 24-h period. The circadian drive promotes sleep at night, and to a lesser extent between 2.00 and 4.00 pm, but at other times it facilitates wakefulness. Superimposed on these drives are reflex and psychological factors which adapt both sleep and wakefulness to changes in the environment of the subject.

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