We report here the development of a brief drug-free rescheduling treatment ("chronotherapy") for Delayed Sleep Phase (DSP) insomnia, a syndrome characterized by sleep-onset insomnia with difficulty in morning awakening. We postulated that patients with DSP insomnia had an inadequate capacity to achieve phase advance shifts of the circadian pacemaker which times the sleep-wake cycle. Chronotherapy was therefore designed to reset these patients' biological clocks by the phase delay route. This single 5-6 day treatment was tested in 5 patients with a 4-15 year history of DSP insomnia. All 5 patients reported a lasting resolution of their symptoms substantiated by systematic long-term self-reports and objective polygraphic recording before and after treatment (average follow-up of 260 days; range, 42-910 days). The average sleep onset advanced from 4:50 a.m. before treatment to 12:20 a.m. afterwards, and wake times advanced from 1:00 p.m. to 755 a.m. (for both, p less than 0.001), with no reduction in sleep efficiency. As a result, all 5 patients were able to end their chronic dependence on hypnotic medications.