Neurobiology of the Human Sleep-Wake Cycle

The human sleep-wake cycle is regulated by two processes: homeostatic process S and circadian process C. Process S is representative of sleep debt, increases during wakefulness and declines during sleep. Adenosine is likely the main neurochemical substrate of process S. Process C is governed by the circadian system. Suprachiasmatic nucleus (SCN), the master pacemaker of the circadian system, aligns physiological and behavioral processes including, sleep-wake cycle to the rotation of Earth. Process S interacts with process C to assure sleep-wake homeostasis. While sleep length and depth correlates with the amount of prior wakefulness (process S), the timing of sleep is mainly influenced by the circadian process C.