The Physiology of Sleep
Sleep influences all major physiologic systems, including thermoregulatory, musculoskeletal, endocrine, respiratory, cardiovascular, gastrointestinal, the nervous system, and the immune system.
Sleep and Thermoregulation
Core body temperature is regulated by the circadian rhythm.1 In normal sleepers and under normal conditions, sleep onset occurs as body temperature falls. Body temperature continues to fall during sleep until about 4:00AM, when it begins to rise again. Humans typically awake during this rising part of the rhythm.
Sleep and the Musculoskeletal System
Most muscles relax during NREM sleep and become atonic during REM sleep (except for the ocular muscles and the diaphragm). This atonia prevents us from moving during sleep, but it can also contribute to the etiology of certain sleep disorders.
Sleep and the Endocrine System
The response of the endocrine system to sleep is complex.2 The secretion of certain hormones (e.g., growth hormone, prolactin, and luteinizing hormone) is increased during sleep, whereas the secretion of others (e.g., thyroid stimulating hormone and cortisol) is inhibited. Some hormones are directly tied to a stage of sleep (e.g., growth hormone is associated with SWS), others to the circadian rhythm (e.g., cortisol peaks in late afternoon regardless of sleep status), or to the darkness/light cycle (e.g., melatonin is released in the dark, and is inhibited by light).3 Considerable research links endocrine dysfunction with sleep dysfunction. It has been proposed that hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, perhaps in response to stress, with subsequent elevations in cortisol and norepinephrine, may inhibit normal sleep function.4
Sleep and the Immune System
There is considerable bidirectional interaction between sleep and the immune system. Both TST and SWS are increased during an acute infection,5 and sleep, especially sleep loss and its contribution to acute stress, is a challenge to the proper functioning of the body’s defense systems.5,6 Conversely, severe illnesses, discomfort, and pain, can inhibit SWS and therefore delay healing.7 Many factors that regulate the immune response have also been shown to modulate sleep, especially NREM. The best data clearly documents bidirectional interaction between NREM sleep and both interleukin-1β (IL-1) and tumor necrosis factor-α (TNF),8 but many similar interactions occur between factors of the immune system and parts of the sleep architecture.