DSIP

Delta sleep-inducing peptide (DSIP) is a nonapeptide first described in the 1970s with the sequence Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu. Reviews still describe it as "a still unresolved riddle": the endogenous precursor is unknown, the receptor system is undefined, and the physiological role is incompletely established despite decades of work.

Mechanistic claims are heterogeneous. A human enzyme immunoassay study reported a very short plasma half-life of about 4.0 ± 0.7 minutes. Separate animal studies suggest indirect modulation of opioid signaling via met-enkephalin release, effects on α1-adrenergic receptor-associated pineal signaling, and inhibition of somatostatin release. The breadth of proposed signals — combined with the absence of a single validated receptor — is one reason the field remains unresolved.