Changes with age

Changes in basic aspects of sleep are prominent from birth to old age, although individual differences are seen at all ages. As in other biological variables, any one sleep measure at a given age will show a relatively normal (bell-shaped) distribution. Changes of clinical significance include the following.

• Total sleep time decreases with age. Average daily values are as follows: newborns, 16 to 18h; young children, 10h; adolescents, 8h; adults, 7.5h and possibly less than this in elderly people. This total amount of sleep includes daytime napping in children up to the age of 3years and often again in old age.

• NREM sleep shows an overall decrease across the lifespan. SWS is particularly prominent in young children who sleep very soundly. Its decline begins in early adolescence and continues throughout adulthood.

• The proportion of REM sleep declines from 50per cent or more of total sleep time in the newborn (more than this in premature babies) to 20 to 25per cent by 2years. This figure remains fairly constant throughout the rest of life. The high level of REM sleep in very early life suggests a role in cerebral maturation but at present its true significance remains unclear.

• NREM-REM sleep cycles occur at intervals of 50 to 60 min in infants who often enter REM at the start of their sleep period. This interval between sleep cycles remains until adolescence when the periodicity changes to 90 to 100 min, which persists into adult life. The amounts of NREM and REM in each sleep cycle is about equal in early infancy. Afterwards NREM sleep (especially SWS) predominates in the earlier cycles and REM sleep in the later cycles.

• Continuity of sleep is greatest in early childhood (as mentioned previously) and least at the extremes of age. Infants are easily wakened and so are the elderly who also wake spontaneously more often. Fragmentation of sleep by brief arousals without actual waking is described as particularly common in the elderly (see Chapter...4.14.5) as confirmed in a recent cross-sectional study which also showed a gradual increase in brief arousals in healthy subjects from teenage to old age.(12)

• Circadian sleep-wake rhythms change considerably in early development. Full-term neonates show 3- to 4-h sleep-wake cycles. Sleep periods have largely shifted to the night and wakefulness to daytime by 12 months, except for napping which gradually diminishes and has usually stopped by about 3 years of age.

However, a physiological tendency towards an afternoon nap remains throughout the rest of life. Although repeated brief waking at night is more common in infancy and early childhood than later, it remains a normal occurrence throughout life increasing in frequency again in old age. The clinical problem arises when there is difficulty returning to sleep after such awakenings.

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