General aspects of normal sleep

In this section, following some general points about the nature and functions of sleep, the main aspects of normal sleep in adults will be described as the basis for a comparison of children's sleep. The emphasis is placed on findings of clinical significance. More detailed information can be found in, for example, the textbook of adult sleep disorders medicine by Kryger et al. (2000).

The nature of sleep

Sleep is a reversible state of reduced awareness of, and responsiveness to the environment. Usually it occurs when lying down, quietly, with little movement.

Sleep is physiologically distinct from other states of relative inactivity such as coma, stupor or hibernation. Within sleep two physiologically distinct states have been defined: Non-Rapid Eye Movement (NREM) sleep and REM sleep.

Both these forms of sleep are active processes. Wakefulness is maintained by cortical noradrenaline, dopamine and acetylcholine release from terminals of brainstem neurones. Activity in the ascending reticular activating system must diminish for sleep to occur. In addition, however, NREM sleep depends on activity in the basal forebrain systems especially, while mechanisms in the pons are primarily responsible for the control of REM sleep. Serotonin and gamma-aminobutyric acid (GABA) neurones are involved in NREM sleep; acetylcholine is essentially involved in the generation of REM sleep. Recent advances in functional neuroimaging are starting to provide further insights into the brain mechanisms of human sleep (Maquet, 1999).

There is no single sleep-promoting substance within the nervous system.