(A basic description of Figure 1 on page 233 is listed with the figure. This is a more detailed description.)
Filled symbols refer to the older subjects; open symbols to the young subjects; means = standard errors. Data are double plotted and shown with respect to circadian phase (lower axis) derived from core body temperature data (nadir temperature waveform = 0¤). The corresponding time of day under normal conditions for the older subjects is shown in the upper axis. Lower panel: percent of wakefulness during scheduled sleep. Middle panel: subjective early awakening. Upper panel: normalized cognitive performance.
There are prominent circadian variations in objective and subjective sleep quality and performance, with all showing a nadir at the circadian phases corresponding to the early morning hours. Older subjects show greater objective and subjective sleep disruption at all circadian phases, and there is a much narrower range of circadian phases when older subjects can remain asleep; young subjects can maintain high sleep quality for many hours after their typical wake time, while older subjects experience increasing levels sleep disruption when scheduled to sleep at or just after the time of their circadian temperature nadir (0¤, on average at 5:15 a.m.). This is also reflected in the circadian performance rhythm, where young subjects show impaired performance at and just after the circadian phases corresponding to their usual wake time, while the performance of the older subjects is improving at these phases.
These measures of sleep and wake indicate that there is a change in the interaction between the circadian timing system and sleep with age, and that there appears to be a decreased drive for sleep in older subjects in the early morning hours.
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JEANNE F. DUFFY CHARLES A. CZEISLER
CLINICAL NURSE SPECIALIST
See NURSE PRACTITIONER