Sleep

After middle age, there is a decline in the duration of SWS, especially in men, from 20 percent to 5 percent or less of total sleep time. There is some debate as to whether this change reflects a significant physiological process, or whether it reflects the inappropriateness of using the standard criteria to define SWS in older people. Amounts of stage 1 sleep increase at this age, perhaps reflecting more nighttime arousals and lighter sleep. REM duration remains relatively constant after early childhood, but its timing changes with age. After about fifty years of age, there is a shortening of the latency (delay from sleep onset) to the first REM episode of the night, perhaps associated with the reduction in SWS durations early in the night.

Although the total duration of sleep may not change dramatically, older people redistribute their sleep throughout the twenty-four-hour day-night cycle. Naps increase in frequency, and sleep during the night tends to become more fragmented and interrupted by longer periods of waking. In contrast to healthy young adults, healthy elderly people may spend only 80 percent of their bedtime at night asleep. Older people also show a preference for both earlier bedtimes and awakening times. It remains unclear to what degree increased napping reflects reduced social pressure to stay awake, compensation for disrupted sleep at night, or a spontaneous change in the daily rhythms of sleep.

An internal daily (circadian) clock regulates the expression of daily rhythms, including the rhythm of sleep and waking. Changes in clock function with aging, including reduced strength (amplitude) of the circadian signal and disrupted rhythm organization, may contribute to changes in sleep habits. These alterations may reflect anatomical changes in the hypothalamic mechanisms that are responsible for circadian rhythm generation.