LATER ENDOGENOUS CIRCADIAN TEMPERATURE NADIR RELATIVE TO AN EARLIER WAKE TIME IN OLDER-PEOPLE

The contribution of the circadian timing system to the age-related adv ance of sleep-wake timing was investigated in two experiments. In a co nstant routine protocol, we found that the average wake time and endog enous circadian phase of 44 older subjects were earlier than that of 1 01 young men. However, the earlier circadian phase of the older subjec ts actually occurred later relative to their habitual wake time than i t did in young men. These results indicate that an age-related advance of circadian phase cannot fully account for the high prevalence of ea rly morning awakening in healthy older people. In a second study, 13 o lder subjects and 10 young men were scheduled to a 28-h day, such that they were scheduled to sleep at many circadian phases. Self-reported awakening from scheduled sleep episodes and cognitive throughput durin g the second half of the wake episode varied markedly as a function of circadian phase in both groups. The rising phase of both rhythms was advanced in the older subjects, suggesting an age-related change in th e circadian regulation of sleep-wake propensity. We hypothesize that u nder entrained conditions, these age-related changes in the relationsh ip between circadian phase and wake time are likely associated with se lf-selected light exposure at an earlier circadian phase. This earlier exposure to light could account for the earlier clock hour to which t he endogenous circadian pacemaker is entrained in older people and the reby further increase their propensity to awaken at an even earlier ti me.