Sleep is a naturally recurring state characterized by reduced or lacking consciousness, relatively suspended sensory and non-motor activity – inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, but it is more easily reversible than hibernation or coma. Sleep is a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish.
The purposes and mechanisms of sleep are only partially clear and are the subject of intense research. Sleep is often thought to help conserve energy, but actually decreases metabolism only about 5-10%. Hibernating animals need to sleep despite the hypometabolism seen in hibernation, and in fact they must return from hypothermia to euthermy in order to sleep, making sleeping “energetically expensive.”
Sleep medicine is a medical specialty or subspecialty devoted to the diagnosis and therapy of sleep disturbances and disorders. From the middle of the 20th century, research has provided increasing knowledge and answered many questions about sleep-wake functioning. The rapidly evolving field has become a recognized medical subspecialty in some countries. Dental sleep medicine also qualifies for board certification in some countries. Properly organized, minimum 12-month, postgraduate training programs are still being defined in the United States. In some countries, the sleep researchers and the doctors who treat patients may be the same people.
The first sleep clinics in the United States were established in the 1970s by interested doctors and technicians; the study, diagnosis and treatment of obstructive sleep apnea were their first tasks. As late as 1999, virtually any American doctor, with no specific training in sleep medicine, could open a sleep laboratory.