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.

Sleep, cognition, and behavior have all been observed to change as a consequence of the normal aging process as well as the pathological processes that occur in Alzheimer's disease (AD). In the early stages of AD, it is quite possible that sleep disturbances that are occurring as a result of normal aging also exacerbate the cognitive and behavioral symptoms of AD. As the disease progresses, sleep disturbances associated with the pathological neurodegeneration characteristic of AD emerge, with further behavioral and cognitive consequences. Sleep disturbances in AD have been noted using many subjective and objective measures, and these sleep disturbances increase with the severity of dementia. Evidence of phase disturbance of the circadian rhythm in patients with AD comes from many different studies using several methods. A recent study demonstrated that treatment compliance with continuous positive airway pressure device (CPAP) could be achieved in patients with possible or probable AD.