Delta Sleep Inducing Peptide (DSIP) is a nonapeptide that is found in neurons, peripheral organs, and plasma. DSIP induces mainly delta sleep in mammals. In addition to sleep, the peptide has been observed to affect electrophysiological activity, neurotransmitter levels in the brain, circadian and locomotor patterns, hormonal levels, psychological performance, and the activity of neuropharmacological drugs including their withdrawal.