Figure A1. Regulation of sleepiness by two biological processes.

A circadian process (red) produces a rising pressure for wakefulness, which counteracts fatigue, during the day; and a withdrawal of that pressure for wakefulness, thereby promoting sleepiness, during the night. Simultaneously, a homeostatic process (blue) builds up a pressure for sleep, thereby promoting sleepiness, during periods of wakefulness; and dissipated that pressure for sleep (dark gray), thereby providing recovery, during periods of sleep (top panels).¹³¹ The combined effect of the circadian and homeostatic processes on sleepiness may be calculated as the net difference between the homeostatic pressure for sleep and the circadian pressure for wakefulness (bottom panels, green),¹³² as illustrated here for a scenario with a daytime duty period (light blue) and an 8-h nighttime sleep opportunity (left panels) and for a scenario with a nighttime duty period (light blue) with an 8-h daytime sleep opportunity (right panels). Note that in the day work scenario, the sleep opportunity is ended (e.g. through use of an alarm clock) somewhat prematurely, as there is still some homeostatic pressure for sleep left to be dissipated (top left). In the night work scenario, however, sleep is curtailed much more, with the rising circadian wake pressure causing awakening from daytime sleep well before the end of the sleep opportunity. The early awakening leaves a higher level of homeostatic sleep pressure at the end of the shortened sleep period (dark gray) and causes a portion of time available for sleep in this scenario (light gray) to remain unutilized (top right). The combined effect of the two processes—sleepiness calculated as the net difference between the homeostatic pressure for sleep and the circadian pressure for wakefulness—is that sleepiness is low and stable throughout the duty period in the daytime duty scenario (bottom left), whereas sleepiness increases and peaks toward the end of the duty period in the nighttime duty scenario (bottom right). Note that the transient cognitive impairment immediately after awakening known as sleep inertia¹¹⁵ is not depicted in this figure.