Delta sleep-inducing peptide (DSIP) is a naturally occurring nonapeptide (Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu) first isolated in 1977 from the cerebral venous blood of sleep-deprived rabbits and subsequently synthesized for research (CAS 62568-57-4; molecular formula C₃₅H₄₈N₁₀O₁₅; molecular mass 848.82 g/mol; PubChem CID 68816). DSIP readily crosses the blood–brain barrier and has been extensively studied, primarily in Russia, Switzerland, and the United States during the 1980s–2000s, for its ability to promote physiological slow-wave (delta) sleep, normalize disturbed sleep patterns, exert stress-protective and adaptogenic effects, reduce chronic pain and withdrawal symptoms in opioid and alcohol dependence, modulate ACTH–cortisol release, lower oxidative stress, and exhibit anticonvulsant, antidepressant-like, and geroprotective properties. Clinical trials have explored its efficacy in primary and secondary insomnia, narcolepsy–cataplexy syndrome, chronic pain syndromes, migraine prophylaxis, menopausal sleep disturbances, and as an adjuvant during opioid detoxification, while preclinical work has demonstrated neuroprotective effects in cerebral ischemia, regulation of circadian rhythms, and enhancement of physical and mental performance under stress. Despite promising early results and decades of use in Russian integrative medicine, DSIP remains investigational in most countries due to inconsistent replication in large Western trials and limited modern mechanistic studies.