This chapter shows that circadian rhythms have a tendency to become less robust with increasing age, i.e. they generally exhibit decreases in amplitude and less stability. Some obvious consequences of such reductions in rhythm amplitude are fragmentation of rhythms, complete loss of temporal order and structure, loss of stability of entrainment, and responsiveness to zeitgebers. Other consequences include changes in clock period and its stability, and inappropriate phase relationships among behavioral and metabolic oscillations. Age-related decreases in the amplitude of circadian rhythms in humans and other mammals have been linked to a deterioration of rhythmic behaviors such as those seen in locomotor activity, feeding and drinking. Anatomical and electrophysiological studies have shown that age-related changes occur within the biological clocks of mammals including humans. There is increasing evidence for age-related changes in the structure and neurochemistry of the suprachiasmatic nucleus (SCN), including alterations in cells producing vasopressin.

This chapter discusses circadian rhythms and the known factors which influence them as well as provide an update on the evaluation and treatments of the six circadian rhythm sleep disorders. The six circadian rhythm sleep disorders include delayed sleep phase type, advanced sleep phase type, non-24-hour sleep/wake syndrome, irregular sleep/wake rhythm, shift work sleep disorder, and jet lag disorder. Zeitgebers such as light are entraining agents of the circadian system and the manipulation of zeitgebers can be used as a therapeutic tool for the circadian rhythm sleep disorders. Measurements of circadian phase can be obtained through various surrogate markers of circadian rhythms, including the core body temperature minimum and the dim light melatonin onset (DLMO). Circadian rhythm sleep disorders are commonly under recognized in clinical practice and should be a part of the differential diagnosis of people who present with symptoms of insomnia and daytime sleepiness.