Delta sleep-inducing peptide (DSIP) is a naturally occurring nonapeptide that was first isolated from the cerebral venous blood of sleeping rabbits in 1977. It is primarily known for its potential role in regulating sleep cycles, particularly promoting slow-wave or delta sleep, and has also been studied for its effects on stress response, pain management, and neuroprotection. Despite extensive research, the precise mechanism by which DSIP works and its specific receptor have not been definitively identified, and clinical trial results in humans have been mixed.