We describe a new syndrome called "delayed sleep phase insomnia." Thirty of 450 patients seen for a primary insomniac complaint had the following characteristics: (1) chronic inability to fall asleep at a desired clock time; (2) when not on a strict schedule, the patients have a normal sleep pattern and after a sleep of normal length awaken spontaneously and feel refreshed; and (3) a long history of unsuccessful attempts to treat the problem. These patients were younger than the general insomniac population and as a group did not have a specific psychiatric disorder. Six patients' histories are described in detail, including the successful nonpharmacological chronotherapy regimen (resetting the patients' biological clock by progressive phase delay). Delayed sleep phase insomnia is proposed to be a disorder of the circadian sleep-wake rhythm in which the "advance" portion of the phase response curve is small.