The study measured a natural sleep‑related peptide called DSIP in the blood of healthy people over a full day. It found that DSIP levels rise during the day, peak around 3 PM, and are lowest around 1 AM, matching the body‑temperature cycle. Surprisingly, DSIP levels drop during deep (slow‑wave) sleep and REM sleep, and they don’t increase right before or during deep‑sleep episodes.

Since delta-sleep-inducing peptide (DSIP) was isolated in 1977, numerous reports have suggested that this nonapeptide stimulates delta-sleep [slow wave sleep (SWS)]. Although DSIP-like immunoreactivity (DSIP-LI) has been found in the serum of many animals and man, its diurnal rhythm and relation to sleep stages have not been well defined. We hypothesized that circulating levels of this putative sleep hormone would be highest at night and would probably be elevated before or during episodes of SWS. We, therefore, measured plasma DSIP-LI levels every 30 min for 24 h in 12 normal volunteers in whom we obtained simultaneous polygraphic recordings. We found a distinct diurnal rhythm for plasma DSIP-LI levels, with the maximum at 1500 h and the minimum at 0100 h. DSIP-LI levels were substantially lower in rapid eye movement sleep (P < 0.005) and somewhat lower in SWS (P < 0.05) compared to awake values. DSIP-LI levels did not rise before, during, or after a significant percentage of episodes of SWS. We found, however, that the diurnal rhythm of DSIP-LI closely followed that of body temperature with a high degree of correlation (r2 = 0.66; P < 0.0001). We conclude that endogenous elevations of circulating DSIP may be associated with suppression of slow wave and rapid eye movement sleep, and that the circadian rhythm of this peptide is coupled directly or indirectly to that of body temperature.