In 1975, the Association for Sleep Disorder Centers -- which, the University of Pittsburgh's Dr. David Kupfer likes to recall, was founded by him and five or six sleep specialists in a motel room in the Chicago airport -- counted five sleep laboratories at major medical institutions in the United States.

Now there are 130. "But because we will do about two site visits a week, the number will surely be greater by the time this is published," said Dr. Merrill Mitler, executive director of the organization, which accredits sleep centers, establishes criteria for sleep disorder diagnoses and sets guidelines for equipment, staffing and records. There may be up to 400 in one stage or another of getting or requesting accreditation. "Suddenly," says Kupfer, "it became a big business."

In these labs, patients either complaining of too much sleepiness in the daytime or inability to fall asleep at night are carefully interviewed and analyzed.

If it appears necessary, they may spend a night or a night and a day in the laboratory hooked to electrodes that measure an assortment of physiologic, sometimes neuroendocrine, activities, brain waves, air flow, eye movements and other body functions and biological cycles.

In addition to identifying problems with sleep and mood disorers, sleep researchers also have provided a way to distinguish between physiological and psychological causes for male impotence. Sexual arousal -- a normal phenomenon during REM sleep -- signifies that impotence while awake is psychological.

Most sleep labs prefer that a patient be referred by his or her own doctor, or at least with a physician's knowledge.

In the Washington and Baltimore area, two sleep labs are fully accredited by the ASDC and two others are in various stages of becoming so. They are:

*Johns Hopkins Sleep Disorders Center, Francis Scott Key Medical Center, 4940 Eastern Ave., Baltimore, Md. 21224; (301) 955-0571.

*National Capital Sleep Center, Inc. (with studies conducted at Suburban Hospital), 4520 East-West Hwy., Suite 406, Bethesda Md. 20814; 656-9515.

*Georgetown University Sleep Disorders Center (awaiting final accreditation), 3800 Reservoir Rd. NW, Washington, D.C. 20007; 625-2697.

*George Washington University Diagnostic Neurophysiological Laboratory and Sleep Studies Center (awaiting accreditation review), 901 23rd St. NW, Washington, D.C. 20036; 676-3210.

For other listings, write: Association of Sleep Disorder Centers, P.O. Box 2604, Del Mar, Calif. 92014.