Impaired daytime functions are a significant part of chronic insomnia besides sleep disturbance. Therefore, the effects of intermediate-term delta-sleep-inducing peptide (DSIP) administration on sleep and daytime performance were investigated in 14 middle-aged chronic insomniacs. DSIP was administered under placebo-controlled, double-blind conditions for 7 successive nights. Polysomnograms were obtained for placebo baseline, beginning and end of DSIP treatment, and one placebo posttreatment night; daytime psychological state and mental performance were extensively tested before and after 6 DSIP injections. The treatment substantially improved night sleep with the first and additionally with repeated doses. These effects were maintained for the first posttreatment (placebo) night. Efficiency of night sleep and daytime rest reached the levels of normal controls. Alertness and performance at daytime increased significantly. The study demonstrates the efficacy of DSIP for the treatment of impaired sleep and daytime functions as well.