Sleep is managed by homeostatic and circadian rhythmic mechanisms. By homeostatic mechanisms, sleep pressure builds up analogous to an inflating balloon throughout the course of the day. By nightfall, sleep pressure builds (balloon inflated) and there is a physiological urge to sleep. Circadian rhythms are repetitive, cyclical changes occurring over a 24-hour period. Hypothalamic nuclei such as the previously mentioned VLPO along with suprachiasmatic nucleus maintain our “biologic clock.” Sunlight and darkness both influence this biologic clock. Typically sleep duration is between 6 to 9 hours for most adults.
The sleep cycle consists of rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. There are 4-6 NREM-REM cycles alternating approximately every 90 to 110 minutes. The first 2-3 cycles feature longer and deeper periods of NREM, while the final 2-3 cycles feature increasing REM periods with shorter and lighter (i.e. less deep) NREM stages. NREM sleep is subdivided into lighter stages (stage 1, stage 2) and a deeper stage 3. It is NREM stage 3 that provides the quality of sleep needed for restoration of sleep-wake physiology and mood (Figure 1).
This chart depicts sleep staging on the vertical axis and the sleep cycle in hours on the horizontal axis. Sleep is deeper in the first half of night than the latter half. W = awake. REM = rapid eye movement sleep. NREM = non-rapid eye movement sleep.