enerally speaking, age is probably the single most crucial factor (apart from the time since the last episode of sleep) that determines how humans characteristically sleep. More so than gender, psychiatric illness and even, to a large extent, most physical illnesses, age is a major determinant of human sleep. Across the human life span, sleep undergoes a wide variety of modifications which are broadly typical of the species, although for any particular aspect of sleep, at any given age, sleep measures have a relatively normal (bell-shaped) distribution.

For example, the amount of nocturnal sleep (7-8 hours) obtained by the majority of adults occupies the mid-point of such a distribution with some individuals obtaining less sleep and some individuals obtaining more sleep. In the age-related patterns of sleep that are discussed below, the data are normative, i.e., they represent group values. Any individual case, of course, may deviate significantly from the mean.


Sleep from Infancy to Adulthood

The newborn infant sleeps about sixteen to eighteen hours per day, and its sleep is widely distributed around the twenty four hour day (Figure 1). This high sleep requirement is assumed to reflect a non-specific restitutional demand that occurs as a result of dramatic growth. By sixteen weeks of age, the total amount of sleep drops to about fourteen or fifteen hours per day and a clear diurnal pattern emerges (Figure 2). This daily sleep quota remains relatively constant for the first year of life. A further gradual decline to about ten to twelve hours occurs between three and five years of age. By age ten, sleep amounts of ten hours or less are reported; sleep then continues to decrease throughout adolescence until the adult pattern is approximated. Paralleling these decreased sleep amounts throughout adolescence are increases in the daytime tendency to fall asleep. Thus, the adolescent decrease in sleep duration may not represent a decrease in sleep need because the decreased sleep duration is accompanied by increased daytime sleepiness.

AS A CHILD DEVELOPS, ITS SLEEP GRADUALLY BECOMES RESTRICTED TO THE NIGHT



FIGURE 1

Polyphasic (multiphase) sleep following birth changes first to biphasic (two-phase) sleep among preschool children and later to monophasic (single-phase) sleep. Among the elderly, periods of sleep during the day become more frequent again.


Sleep in Old Age

Most evidence indicates that as individuals approach old age, the amount of nocturnal sleep decreases; older individuals usually sleep only six to seven hours (Figure 2). Also their sleep is more fragmented by wakefulness and is more susceptible to disruption by noise. One interpretation of these findings is that sleep need is decreased in older individuals. However, inferences about sleep need from sleep duration can be misleading. While decreased sleep is consistent with a decreased need to sleep, decreased sleep need is also consistent with the decreased capacity of the "sleep mechanism" to meet a normal sleep requirements. Some preliminary evidence supports a decreased sleep need in the elderly compared to young adults, but other evidence supports a "normal" or unchanged sleep need in the elderly, compared to younger adults. Thus, on the one hand, in response to sleep deprivation, older individuals show the same compensatory sleep increase as younger individuals. This suggests that older individuals have a reduced sleep need in the presence of a "normal" sleep mechanism. On the other hand, the decreased nocturnal sleep of older individuals may be partially offset by increased daytime napping. As a result, the 24 hour total sleep time of young and old adults may be quite similar. This finding suggests that sleep need in old adults is not less than sleep need in young adults. Thus, the sleep need of older adults remains undetermined and probably will remain so until we know more about the functions of sleep; then we may be able to determine the extent to which sleep fulfills its functions.


Developmental Course of REM Sleep

The quantity of REM sleep (defined as the proportion of total sleep time) may exceed fifty percent in the newborn; premature babies have even higher amounts (Figure 2). REM sleep amount declines throughout the first year of life. By the time a child is about two years of age, REM occupies about twenty to twenty five percent of total sleep, a figure which remains relatively constant throughout childhood, adolescence and adulthood. The reasons for the high levels of REM in the neonate are unclear, though it has been speculated that REM sleep is important for the maturation of the cerebral cortex and the oculomotor system, and that it assists in the programming of developing neuronal circuits.



FIGURE 2

Graph showing changes with age in total amount of daily sleep and percentage of REM sleep. There is a sharp drop in the amount of REM sleep after the early years, falling from 8 hours at birth to less than 1 hour in old age. The change in the amount of NREM sleep is much less marked, falling from 8 hours to about 5 hours over the lifespan.

Ontogenetic changes in REM sleep are not limited to the amount of REM sleep. The alternation of NREM and REM sleep, which occurs at approximately 90-100 minute intervals in middle-aged adults, occurs at fifty to sixty minute intervals in human infants. Infants also may pass directly from wakefulness to REM sleep, thereby bypassing the first NREM cycle at the beginning of the night. Despite these changes, the percentage of REM does not vary appreciably in normal aging (Figures 2 and 3). There are some suggestions that in certain conditions of pathological aging (e.g., Alzheimer's Disease) REM sleep may be reduced and that this reduction may reflect the decline in cholinergic function that occurs in this disorder.



FIGURE 3

Normal sleep cycles in children, young adults, and elderly humans. REM sleep (darkened area) occurs cyclically throughout the night at intervals of approximately 90 minutes in all age groups. REM sleep shows little variation in the different age groups, whereas Stage 4 NREM sleep decreases with age. In addition, the elderly have frequent awakenings and a marked increase in total wake time.


Developmental Course of NREM Sleep

In contrast to REM sleep, Stages 3 and 4 of NREM sleep (i.e., synchronized, delta wave sleep, slow wave sleep) show a steady decrease across the lifespan (Figures 2 and 3). By three to five years of age children typically have abundant, high amplitude, delta wave sleep which is relatively impervious to noise and external disruptions. Beginning in early adolescence, there is a gradual decline in delta wave sleep. Even within college age populations, this age-related decline can be observed. This decline continues throughout adulthood. This decrease in Stages 3 and 4 sleep is generally replaced by NREM Stages 1 and 2. By the time the average human reaches age 75, Stage 4 sleep, the sleep with the most abundant delta waves, may be virtually absent. The functional significance of the age-related decline in human delta activity during sleep is unknown, but it has been proposed that morphological changes related to a loss of neuronal "plasticity" may be involved. At any rate, the decline may represent one of the earliest known indicators of the aging of the central nervous system.


Sleep Pathologies

Most abnormalities of sleep show a characteristic prevalence that is age-related. Night terrors or somnambulism occur in about 3-5% of school-aged children; they decrease in adolescence. Bedwetting occurs in as many as 10% of 4 year olds, but at 8 years of age only about 3% of boys and 1% of girls have a problem with bedwetting. Trouble sleeping through the night is quite common in 1-4 year olds (30%), but it decreases substantially (1-3%) by age 8. The prevalence of poor sleep is also relatively low in adolescence, but when it does occur it is likely to be characterized by difficulty in falling asleep. Narcolepsy, thought to afflict 1 individual in 10,000, also typically begins in adolescence. In adulthood, insomnia is by far the most prevalent sleep disorder; its estimated prevalence increases from about 25% at age 30 to over 50% at age 70. Aged individuals are particularly susceptible to awakening during the night and then being unable to return to sleep (Figure 3). Both periodic leg movements (sometimes called nocturnal myoclonus) and apnea (see below) occur in the sleep of 20-30% of people who are more than 65 years old.

Over the last fifteen years there has been the recognition that respiration is strongly affected by an individual's sleep and waking states. This discovery was fostered by the finding that sleep apnea (sometimes also called sleep disordered breathing or sleep related respiratory disturbance) occurs very frequently at the opposite ends of the human life span, i.e., in both newborn infants and in the aged human adult. In the neonate, prolonged pauses in respiration have been suggested to be one possible mechanism underlying the Sudden Infant Death Syndrome (SIDS). This syndrome tends to run in families. Although the evidence is inconclusive, deficient respiratory control during sleep and/or deficient arousal mechanisms are thought to be involved. In the aged, apnea has been speculated to be related to "natural" death occurring during sleep, although the evidence for this hypothesis is only now beginning to accumulate. It is also likely that sleep apnea in old age represents a different constellation of risk factors from apnea when it occurs in infancy.

Proceed to Part D.

Return to Syllabus Home.