Sleep

Another question which remains only partially answered is how sleep onset is determined and why. The factors involved include circadian rhythms (biological time clocks); the degree of stimulation in the wakeful state; the degree of personal sleepiness; the decrease in core body temperature; a quiet and comfortable sleep environment; conditioning arising from "bedroom cues"; and homeostasis, the automatic attempt by the body to maintain balance and equilibrium (for example, the air temperature may fall to 50°F [10°C], but our body burns calories to maintain its normal temperature of 98.6°F [37°C ]).

The fact that sleep deprivation increases the desire for sleep firmly points to a homeostatic element in sleep. This is intricately linked to highly influential circadian rhythms controlled by centers probably located in the hypothalamus, part of the brain primarily involved in autonomic nervous system functions. Circadian rhythms determine our approximate 24- to 25-hour sleep-wake pattern and a similar cycle in the rise and fall of core body temperature and other physiological functions.

It is not yet known whether two separate biological clocks influence sleep-wake cycles and temperature levels and, if so, if a single "control clock" regulates them both. However, body temperature drops slightly in the evening as sleep draws near, reaches its lowest point around 2:00-4:00 A.M., rises slightly before awakening, and increases to maximum as the day progresses. This pattern is not a result of being asleep or awake, for body temperature does not drop during daytime naps nor does it rise at night after a sudden change in sleep schedule, such as shift work. It takes about two weeks for circadian rhythms controlling temperature levels to get back into sync with sleep-wake states.

Studies done on human circadian rhythms in situations totally devoid of time cues (such as sunrise, sunset, clocks, etc.) show that these rhythms are controlled completely internally and usually run on a cycle of almost 25 rather than 24 hours. In normal situations, factors called "zeitgebers" (from the German zeit for time and geber for giver) such as daylight, environmental noises, clocks, and work schedules virtually force us to maintain a 24-hour cycle. Therefore, our circadian rhythms must "phase advance" from their normal, approximate 25-hour cycle to an imposed 24-hour cycle.

The body has difficulty adapting to much more than an hour of phase-advance in one day. Drastic time changes-like those caused by rapid long-distance travel such as flying-require either phase-advancement or phase-delay. This is why travelers experience "jet lag." Recovery from east-west travel requiring phase-delay adjustments is usually quicker than in phase-advancement resulting from west-east travel. Some people seem simply unable to phase-advance their biological clocks, which often results in sleep disorders.