Sleep physiology

Profound changes take place during childhood in basic sleep physiology although many are complete by about 6 months of age. In general, there is a progression towards differentiation and organization of conventionally defined sleep states, shorter sleep time, less napping, less slow-wave sleep ( SWS), and longer sleep cycles.

Specific aspects of particular clinical importance are as follows.

• Typical sleep duration (including naps) at different ages is shown in Tablel..

f gkem birth 16h

îyears 13li

S years 11 Ii iytars 10 h

14yars 9 h

17 yews 6 h

Table 1 Average sleep duration at different ages

• Sleep usually becomes confined to night-time by 3 years.

• Rapid eye movement (REM) sleep is prominent in early infancy possibly explaining why sleep is fragile then.

• In comparison, by early childhood SWS is particularly pronounced. This predisposes children of that age to arousal disorders which arise from SWS. SWS is not reduced in sleep disorders (e.g. obstructive sleep apnoea ( OSA) which in adults leads to a decrease in this stage of sleep). This might explain the usual absence of extreme degrees of daytime sleepiness in children of that age with these disorders.

• Between about 5 years of age and puberty, overnight sleep is especially sound and alertness is maximal during the day. Various conditions causing excessive daytime sleepiness in adults may well not have this effect in children because of this increase in alertness. However, overnight sleep may become extended.

• In contrast, adolescence is characterized by an increase in daytime sleepiness. The amount of SWS decreases, the sleep phase is physiologically delayed, and with the onset of puberty there is no longer the decrease in physiological sleep requirements seen progressively at earlier ages. The combination of these factors and strong social influences to stay up late, especially at weekends, frequently causes erratic sleep-wake schedules and daytime sleepiness.

Sleep continuity in children has been studied very little. Awakenings during the night are normal at any age although they are often not recalled unless there is difficulty returning to sleep. Recent normative data from home polysomnography indicate only a tendency for the number of awakenings to increase with age in children between 5 and 16 years.(3) Microstructural arousals are even less well described at any age in normals despite their apparent clinical significance.

There are indications that, following sleep deprivation, sleep physiology takes longer to return to normal in children than in adults. In one of the few experimental studies of sleep restriction in children, the usual rebound increase of SWS, followed by REM sleep, took several nights compared with the one to two nights in adults.

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