Circadian Rhythms

One prominent age-related change in the organization of daily behavior is the advance of bed and wake times to an earlier hour. Associated with this is an age-related increase in two specific sleep complaints, early morning awakening and difficulty maintaining sleep. Early morning awakening and difficulty maintaining sleep in the latter part of the night have been shown in a number of studies to affect up to 40% of the older population. It has also been reported that age-related sleep disturbances are associated with increased mortality and with increased usage of sleeping pills.

Given the role played by the circadian system in sleep-wake timing, age-related changes in circadian rhythms have been hypothesized to underlie the shift of sleep-wake timing and the increase in circadian rhythm sleep disorders in older people. Numerous reports that daily physiologic rhythms occur at an earlier hour in older people than in young adults seem to support the idea that there is a change in the circadian timing system with age. In fact, circadian entrainment theory predicts that a shortening of the period of the circadian pacemaker would result in a phase advance of circadian rhythms with respect to the light-dark cycle, the main environmental signal synchronizing circadian rhythms to the 24-hour day. Studies of animal circadian rhythms carried out in the 1970s and 1980s supported the idea that the period of the circadian system shortened with age. Those studies compared separate groups of young and older animals. In the 1990s, studies were carried out in which the circadian period of animals was monitored throughout their entire life span. Those more recent studies found that the average period in when the animals were old was not significantly different from when the same animals were young, thus refuting the idea that circadian period shortens with age.

Early studies of human circadian rhythms had also suggested that circadian period shortened with age, but those studies were confounded by allowing the subjects to self-select their sleep-wake and light-dark times. During the 1990s, a series of forced desynchrony studies were carried out in very healthy young and older human subjects to compare circadian period lengths between the two age groups. Those studies found no significant difference in circadian period with age. Thus, a key feature of the circadian timing system, its intrinsic period, does not appear to change with age.