Six patients, all but one, men, between the ages of 65 and 77 years, were regularly awakened from sleep at a consistent time of night, sometimes during a dream, by a diffuse headache that persisted for 30 to 60 minutes and was often associated with nausea, without autonomic symptoms. In all cases, headache ceased or was substantially attenuated by a bedtime dose of 300 to 600 mg of lithium. The disorder persisted for one and a half to three years in three of the patients; the others continue to require lithium. It is proposed that this syndrome may be caused by perturbation of a biological clock.