This paper summarizes different investigations into effects of delta sleep-inducing peptide (DSIP) injections on insomnia. Two different studies showed improvement of sleep following single injections of 25 nmol/kg b.w. before sleep. Repeated administrations indicated a buildup with normalization of sleep structure after four administrations. Repeated injections in the morning – besides increasing daytime activity – still had a strong positive effect on night sleep, but not so two doses daily. A case of insomnia in organic brain disease responded well to higher doses. The results are discussed as to the mode of action of DSIP and its possible therapeutic use in insomnia.