Primary or psychophysiological insomnia has alternatively been viewed as either a predominantly psychological problem or as a predominantly physiological problem. Several early studies of patients were not able to document physiological differences, but more recent studies have found that many patients with primary insomnia take longer than control subjects to fall asleep on daytime nap tests despite feeling fatigued and they have elevated metabolic rate throughout both night and day. Other recent studies have found that increasing physiological arousal level for a week in normal sleepers produced the major secondary symptoms reported by insomniacs. In contrast, producing the disturbed sleep of insomniacs in a group of normal sleepers did not produce the typical pattern of secondary symptoms. Taken together, evidence is presented which supports the contention that primary insomniacs suffer from a disorder of hyperarousal and that the elevated arousal produces the poor sleep and other symptoms reported by patients. It is therefore suggested that new treatment strategies directed at reduction of arousal level be considered in these patients.