Sleep is as essential to health as air, food, and water. Human performance, in everyday life and in sports, will depend on the quality and the regularity of sleep. From a technical perspective, sleep may be defined as the natural state of rest where the person sustains a partial or complete loss of consciousness. The body during sleep is less responsive to external stimuli, and the brain activity is altered during sleep, where the brain sometimes engages in dreams.

Stanford University student being monitored for heart rate and temperature during a flight from California to Tokyo as part of a study on jet lag.

The primary purposes of sleep are to rest the body, to permit the repair of musculoskeletal tissues, and for the body to generally recover from the stresses imposed on it during the waking hours. The amount of sleep that an individual requires will vary from person to person; as a general proposition, an adolescent will requires over nine hours of sleep per day, a consequence of their body's growth. An adult typically requires between seven and nine hours of sleep per night. Persons who are active in sports will generally require a greater quality, if not quantity, of sleep each night.

The effectiveness and the duration of sleep is governed by two distinct factors. Sleep/wake homeostasis is balance that the body strives to maintain between its wakefulness and sleep as a restorative process. When the body has been subjected to a very demanding period of activity, it will seek to have the sleep period be one of corresponding quality and duration to achieve homeostasis (balance).

The second factor governing sleep patterns is the circadian biological clock, sometimes referred to as the body's circadian rhythms. The circadian biological clock tells the body when it is most alert and when it is required to sleep. These rhythms are governed by the hypothalamus, the region of the brain that governs a diverse group of functions, including body temperature, hormonal release, and how the body responds to light and darkness. Most adults experience their most powerful desire to sleep between 2:00 a.m. and 4:00 a.m., with a lesser rhythm present between 1:00 p.m. 3:00 p.m. daily.

Circadian rhythms are not constant; external factors such as prior sleep quality and jet lag will disrupt the rhythms. The desire to nap during the course of a given day is tied in part to the presence of the rhythms; the afternoon siesta that is a part of the culture of many warm-weather countries is consistent with these natural rhythms.

Sleep can be disrupted by a multitude of conditions, some of which are a consequence of physical illness or disease, others more transient and often environmental in nature. Snoring, which disrupts the quality of sleep for both the snorer and any sleep partner, can be caused by a number of physical factors; overweight persons tend to snore more frequently. Medications, particularly those that have a steroid in their formulation, may disrupt sleep. Caffeine consumption (or other stimulants) artificially counters the effects of fatigue, the body's natural signal to rest. Alcohol is a central nervous system depressant, creating an artificial desire to sleep and a disruption of the body's cycle.

Jet lag is the well-known expression for the effect on the circadian biological clock caused by travel to different global time zones. Jet lag can be particularly disruptive to the achievement of peak athletic performance. The athlete's body, which may be accustomed to afternoon or evening performance times, is essentially tricked by the imposition of a lengthy time difference. Jet lag generally affects athletes who are competing in a time zone more than four hours apart from their home time zone.

Restless leg syndrome is a nervous system condition where the legs move spasmodically through sleep, affecting sleep quality. Another condition, sleep apnea, is a breathing disorder that creates temporary obstructions of breath as the sleeper inhales and exhales. In a rare worst-case scenario, sleep apnea could lead to a cardiopulmonary disruption; sleep apnea contributes to poor quality sleep. Nocturia is a need to urinate at night; a number of kidney disorders and medication side effects will disrupt the sleep of persons in this fashion.

Sleep inertia is a condition that may arise in persons who enjoy a full night's sleep. In some persons, sleep inertia can create a state where the person suffers from an impairment of the cognitive abilities that may last as long as two hours upon awakening. The portion of the brain responsible for the formulation of plans and the solving of problems, the prefrontal cortex, does not reach an active level at the same speed as other parts of the brain. Caffeine, through coffee, is the typical manner in which people counter the effects of sleep inertia.

SEE ALSO Exhaustion; Fatigue; Fitness; Health; Sleep deprivation and sports performance.