DSIP is a chain of nine amino acids that a Basel group isolated in 1977 from the blood leaving rabbit brains and first regarded as a natural signal that promotes deep sleep. In the decades since, no gene, precursor protein, or receptor for it has been identified, so how it would act in the body is still unknown. Its structure matches no known peptide family, and one review proposes that a related, still unidentified peptide may explain the DSIP-like activity found in the brain. Early animal work suggested it might act on the same brain systems as opioids, which is why it was tried in withdrawal.
In animals it disappears from the blood within minutes, with a half-life of 3 to 6 minutes in dogs and about 2 to 3 minutes in rats and a monkey. Some of it crosses into the brain, partly through the choroid plexus, the tissue that makes spinal fluid. In the insomnia trials, its effects on sleep measured overnight were small and inconsistent from one study to the next. Because it is a peptide given by injection, the FDA flags a risk that the immune system could react to it or to impurities left from making it.
