The Sleep–Wakefulness Cycle
Wakefulness (W) is necessary for a thoughtful and precise knowledge of things, allowing us to recognize our essential attributes and the changes that we experience in ourselves. We spend about two-thirds of our life in W. This state is circadian and homeostatically regulated and precisely meshed with sleep into the sleep–wakefulness cycle (SWC). Sleep is also a necessary, active, periodic, and diverse condition. Although five different stages have been described for sleep in man, most experimental studies have curtailed them into two sleeping stages: Slow wave sleep (SWS), also called non-REM sleep (NREM sleep); and rapid eye movement sleep (REM sleep or paradoxical sleep). Together with W, these three phases constitute the SWC. The hypothalamic suprachiasmatic nucleus is the pacemaker for SWC circadian rhythmicity. Photic retinal stimulation by light modulates suprachiasmatic nucleus activity through the retino-hypothalamic pathway, tuning the SWC to a circadian rhythm with a nocturnal sleep time in adult humans.
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