Age-related reduction in the maximal capacity for sleep--implications for insomnia

Abstract

Sleep changes markedly across the life span and complaints about insomnia are prevalent in older people [1]. Whether age-related alterations in sleep are due to modifications in social factors, circadian physiology, homeostatic drive, or the ability to sleep remains unresolved. We assessed habitual sleep duration at home and then quantified daytime sleep propensity, sleep duration, and sleep structure in an inpatient protocol that included extended sleep opportunities covering 2/3 of the circadian cycle (12 hr at night and 4 hr in the afternoon) for 3-7 days in 18 older and 35 younger healthy men and women. At baseline, older subjects had less daytime sleep propensity than did younger subjects. Total daily sleep duration, which was initially longer than habitual sleep duration, declined during the experiment to asymptotic values that were 1.5 hr shorter in older (7.4 +/- 0.4 SEM, hour) than in younger subjects (8.9 +/- 0.4). Rapid-eye-movement sleep and non-rapid-eye-movement sleep contributed about equally to this reduction. Thus, in the absence of social and circadian constraints, both daytime sleep propensity and the maximal capacity for sleep are reduced in older people. These data have important implications for understanding age-related insomnia.

Figure 1

Protocol and sleep times from one subject. Data are plotted in raster format in which time in hours is plotted horizontally and day number is plotted vertically; two consecutive days are presented on each row. The timing of sleep, as derived from actigraphy and self-report while the subject was at home, is indicated by open boxes. Inpatient sleep episodes are indicated by black boxes and Multiple Sleep Latency Tests by gray boxes. In the present report, we include the data collected through the sleep extension segment of the protocol; data from the second set of MSLTs and following sleep episode are not included.

Figure 2

Daytime sleep propensity as assessed by the Multiple Sleep Latency Test. (a) Survival curves of sleep latency, i.e. the percentage of subjects who are awake for that wake duration or shorter is plotted against elapsed time during 20-minute nap opportunities. Data of all five nap opportunities for each subject are included. (b) Histograms of percentage of older and younger subjects by median sleep latency. (c) Histograms of percentage of subjects by median sleep latency separately for subjects with a Habitual Sleep Duration less than 7 hours, between 7 and 9 hours and greater than 9 hours.

Figure 3

a) Total Sleep Time (TST) during each day (E1-E7) of increased sleep opportunity, and during the 12-hour nighttime and the 4-hour daytime sleep opportunity within each day. b) TST, Rapid-Eye Movement (REM) sleep and non-REM (NREM) sleep during the baseline sleep episode (Day BL, shaded area) and during days E1-E7 for older (filled symbols) and younger (open symbols) subjects. The data are plotted as mean and standard errors estimated by the fitted exponential function.