Modelling Sleep Propensity and Sleep Disturbances

Sleep propensity (SP) can be defined as the readiness to transit from wakefulness to sleep, or the ability to stay asleep if already sleeping. The time course of SP within 24 hours shows several distinct features. The predominant temporal distribution of human sleep is characterized by the occurrence of one long sleep episode within each circadian cycle. This pattern is typical for entrained and free-running conditions (Czeisler et al., 1980; Zulley et al., 1981). It has been shown that the strength of the monophasic sleep—wake distribution depends on the strictness of the sleep—wake schedule and that additional, shorter sleep episodes appear with a greater chance if the experimental or social restrictions to fall asleep are low (Campbell, 1984). The most frequently observed deviation from a monophasic sleep—wake distribution is the occurrence of an additional sleep episode in the afternoon, halfway between two episodes of night sleep (Soldatos et al., 1983; Campbell, 1984; Lack and Lushington, 1996). Although napping is the most obvious indicator for systematic fluctuations of vigilance during daytime hours, additional evidence comes from circadian studies with performance tests (Minors and Waterhouse, 1981, chapter 6), and from the analysis of the temporal distribution of traffic accidents during 24 hours (Prokop und Prokop, 1955; Lauber and Kayten, 1988).