Regulation of sleepiness: the role of the arousal system.

Sleepiness is a widespread phenomenon in the busy industrial countries, and many studies have identified its significant negative impacts on individuals and society. Particularly important are the data that associate sleepiness with the risk of accidents at workplace and in transport, pointing to shift workers as the most vulnerable population. It is generally accepted that two basic physiological processes regulate sleepiness: homeostatic and circadian rhythmic processes. Recent research has proposed the third component regulating sleepiness, that is, the wake drive or the arousal system. The role of the arousal system in regulating sleepiness has partly been addressed by the studies of the pathophysiology of insomnia, which is often described as a disorder of hyperarousal. Experimental and correlational studies on the relation between sleepiness and arousal in good sleepers have generally indicated that both physiological and cognitive arousal are related to the standard measures of sleepiness. Taking into account the role of the arousal system in regulating sleepiness widens the possibilities for the management of sleep disorders and could also help in solving the problem of excessive sleepiness at work and the wheel.