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 physicians who treat patients may be the same people.

The first sleep clinics in the United States were established in the 1970s by interested physicians and technicians; the study, diagnosis and treatment of obstructive sleep apnea were their first tasks. As late as 1999, virtually any American physician, with no specific training in sleep medicine, could open a sleep laboratory.

Disorders and disturbances of sleep are widespread and can have significant consequences for affected individuals as well as economic and other consequences for society. The US National Transportation Safety Board has, according to Dr. Charles Czeisler, member of the Institute of Medicine and Director of the Harvard University Medical School Division of Sleep Medicine at Brigham and Women's Hospital, discovered that the leading cause (31%) of fatal-to-the-driver heavy truck crashes is fatigue related, (though rarely associated directly with sleep disorders, such as sleep apnea), with drugs and alcohol as the number two cause (29%). Sleep deprivation has also been a significant factor in dramatic accidents, such as the Exxon Valdez oil spill, the nuclear incidents at Chernobyl and Three Mile Island and the explosion of the space shuttle Challenger.

Competence in sleep medicine requires an understanding of a plethora of very diverse disorders, many of which present with similar symptoms such as excessive daytime sleepiness, which, in the absence of volitional sleep deprivation, "is almost inevitably caused by an identifiable and treatable sleep disorder," such as sleep apnea, narcolepsy, idiopathic hypersomnia, Kleine-Levin syndrome, menstrual-related hypersomnia, idiopathic recurrent stupor, or circadian rhythm disturbances. Another common complaint is insomnia, a set of symptoms that can have many causes, physical and mental. Management in the varying situations differs greatly and cannot be undertaken without a correct diagnosis.