Effects of light and temperature on sleep in adults and elderly

From the very beginning of evolution on, virtually all life forms have developed in an environment where light intensity and temperature cycled at a rate of about 24 h. It is therefore not surprising that rhythms of about 24 h, i.e., circadian, are integrated in all processes of life, including gene expression, the biochemical processes in a cell, the complex physiology of an organism, its behavior and cognition. A complex system comprising central autonomous oscillators and peripheral slaved damped oscillators is responsible for this circadian modulation of our bodily functions, and is described in detail in other parts of this Encyclopedia. Sleep is one of the states that is under control of the body clock system; sleep is most likely to occur within a specified phase range of the circadian cycle. Homeostasis is the second important factor involved in sleep regulation; the need for sleep increases with the duration of wakefulness. The circadian and homeostatic regulatory systems interact to concertedly regulate the timing of sleep and wakefulness. With increasing age, cracks may start to appear in the functionality of this complex interacting system. For human sleep, this can result in a decreased ability to maintain uninterrupted sleep during the night and an increased probability for napping during the day. Sleep complaints indeed increase with age and are among the most disturbing subjectively judged consequences of getting older. At present, satisfactory pharmacological interventions to improve the sleep–wake rhythm on a daily basis are lacking. The present paper addresses the question whether the human sleep-wake rhythm is still sensitive to its evolutionary oldest environmental modulators or “Zeitgebers”: light and temperature. The focus moreover is on their capacity to support the sleep–wake rhythm of elderly people on a daily basis.