DSIP: why the 'sleep peptide' is still an unresolved riddle

DSIP (delta-sleep-inducing peptide) is a nine-amino-acid peptide first isolated in 1977 from the blood of rabbits in induced sleep. Nearly fifty years later, nobody has identified its receptor or confirmed that it reliably causes sleep. Key papers: 1. Graf & Kastin (Neurosci Biobehav Rev, 1984), an early review. https://doi.org/10.1016/0149-7634(84)90022-8 2. Graf & Kastin (Peptides, 1986), an update two years later. https://doi.org/10.1016/0196-9781(86)90148-8 3. Kovalzon & Strekalova (J Neurochem, 2006), whose title calls DSIP "a still unresolved riddle". https://doi.org/10.1111/j.1471-4159.2006.03693.x For discussion: why have the sleep results been so inconsistent between labs, and has anything newer moved the question forward? The full write-up is in Guides: DSIP.