Researchers found a peptide called delta‑sleep‑inducing peptide (DSIP) or similar material in human breast milk. It’s most abundant right after birth, drops over the first two months, and follows a daily rhythm with higher levels in the afternoon. The peptide appears in larger forms than the pure nine‑amino‑acid version, and it’s unclear whether it actually helps newborns sleep.

Delta-sleep-inducing peptide (DSIP)-like material was detected in human breast milk of two women by RIA with a recovery of about 90%. The high concentration of DSIP-like immunoreactivity (DSIP-LI) in colostrum (30 ng/ml) decreased to about 10 ng/ml in milk. The concentration continued to decrease over the next 2 months in one women. In the same woman, a significant circadian rhythm of the amount of breast milk DSIP was found with the peak in the afternoon and the trough in the morning. A significant effect of the sampling procedure was detected in the other woman examined; lower amounts of DSIP-LI were found when the milk was collected before and higher concentrations after nursing. Gel chromatography revealed that most of the immunoreactive DSIP-LI in milk and colostrum occurred in a form larger than the nonapeptide. The presence of DSIP itself, however, was demonstrated by high pressure liquid chromatography, which also showed additional peptides reacting with the antibody. Digestion of the large immunoreactive DSIP-LI by trypsin produced a peak on Sephadex G-10 that coeluted with DSIP. This peak contained three immunoreactive fractions with retention times on high pressure liquid chromatography similar to DSIP, phosphorylated DSIP, and N-tyrosine-DSIP. Plasma samples taken during pregnancy were assayed for DSIP but no difference from normal values was found. Slightly higher amounts were found in placenta than in blood, which might be due to interfering substances. No Tyr-MIF-1 or corticotropin-releasing hormone was detected by RIA in human breast milk. Peptides and proteins of milk can be absorbed from the gastrointestinal tract of babies, but it is not known if the DSIP-LI in human milk is involved in the induction of a sleep-wake cycle in neonates.