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 apnoea were their first tasks. As late as 1999, virtually any American physician, with no specific training in sleep medicine, could open a sleep laboratory.