The effects of delta sleep-inducing peptide (DSIP), on the sleep cycle of insomniac patients were assessed by means of polysomnographic recordings. DSIP in a dose of 25 nmol/kg or a placebo was administered i.v. during four nights using a double-blind crossover design. The number of nocturnal awakenings, non-rapid-eye-movement (NREM) sleep latency, total waking time and waking time after sleep onset were decreased under DSIP treatment, but no significant differences were found in comparison to baseline or to double-blind placebo nights. Total sleep time and NREM sleep time were increased by the peptide. Their increase was related to increases in stage 2, while stage 1, slow wave sleep (stages 3 and 4) and rapid-eye-movement sleep were not modified. For NREM sleep time and stage 2 sleep differences between DSIP and a placebo were significant, but the same differences existed already for the baseline values. It can be concluded that sleep improvement under DSIP treatment is of little clinical significance.