雑誌掲載版 There are three major neurophysiological mechanisms underlying the sleep-waking cycle: the sleep system, the waking system, and the system that determines sleep-waking timing. Sleep dlisorders of older adults seem to be caused by functional or organic changes in one or more of the three systems, and are roughly classified into two categories: (i) normal age-related, and (ii) pathological. The former includes decreased amplitude and advanced phase of circadian rhythms (body temperature, melatonin secretion, and sleep-waking), as well as reduced sleep duration, sleep fragmentation, and a decrease of slow-wave sleep in sleep architecture. Pathological sleep disorders include medical and psychiatric diseases (e.g., lifestyle-related diseases, dementia, delirium, and depression) and primary age-related sleep disorders (e.g., REM sleep behavior disorder and periodic limb move- ment disorders). This mini-review delineates the clinical features of sleep disorders in older adults.