Circadian Rhythms

Despite consensus on the significance of both the rhythmic circadian and the sleep-wake dependent processes in the regulation of sleep in young adults, attempts to quantify the extent to which each process contributes to sleep in older people are scarce. The forced desynchrony studies of human circadian rhythms described above, in which it was found that circadian period is not significantly different with age, revealed several new findings about the relationship between the circadian timing system and the sleep-wake cycle in aging. Despite the fact that the older subjects in that type of study were extremely healthy, had no sleep complaints, and were screened to rule out the presence of sleep disorders, they slept much more poorly than did the young subjects. On the baseline nights in that study when all subjects were sleeping at their habitual times, the older subjects had a sleep efficiency of only 77 percent compared with a greater than 90 percent sleep efficiency in the young subjects. During the forced desynchrony segment of the study when sleep episodes were scheduled at all different circadian phases, even small changes in the timing of sleep with respect to the phase of the circadian pacemaker resulted in substantial self-perceived and objectively-measured sleep disruption. This sleep disruption was greater in the older subjects, especially when the latter part of the sleep episode was scheduled to occur after the temperature minimum, which occurred on average in those same subjects at 5:15 a.m. Finally, there was a much narrower range of circadian phases when older subjects could maintain high sleep efficiency at the end of their scheduled sleep episodes. It therefore seems that the circadian drive for sleep is reduced in the early morning hours in even very healthy older individuals. This finding suggests that there is a crucial relationship between the circadian timing system and the timing of the sleep episode, and that if the alignment between these two regulatory systems is altered, the sleep of older individuals is much more vulnerable to disruption. Two additional studies support the notion that the alignment between the circadian timing system and the timing of the sleep-wake cycle may be altered in aging. Those studies used different experimental designs, but both found that the relative timing of the habitual sleep-wake cycle with respect to the timing of circadian rhythms is significantly different in older subjects.