Delayed sleep phase syndrome (DSPS) comprises a persistent or recurrent pattern of sleep disturbances, sleep disruption that leads to insomnia and/or excessive daytime sleepiness, and impaired functioning in social, occupational, or other spheres. Aripiprazole (APZ), a second-generation antipsychotic, manifests a novel mechanism of action by serving as a partial agonist of both D2 and serotonergic 5-HT1A receptors and antagonist of 5-HT2A receptors. We have used APZ to treat DSPS. One reason it was effective may be that the insomnia induced by daytime APZ was effective in treating the patient's daytime sleepiness. Another reason may be APZ increases histamine release which controls sleep-wake cycles. Thus, APZ may be therapeutic for DSPS.