DSIP breaks down fast in blood, turning into small pieces like tryptophan, and this happens faster at higher temperatures and varies between humans and rats. Two modified versions of DSIP, especially the phosphorylated one, stay intact longer and tend to stick together in the blood, which may let them work for a longer time. This suggests that the quick disappearance of regular DSIP is due to degradation, while the analogs linger because they degrade slower and form complexes.

The biostability of DSIP (delta sleep-inducing peptide) and two analogs in blood was investigated in order to determine if rates of inactivation contribute to variable effects in vivo. Incubation of DSIP in human or rat blood led to release of products having retention times on a gel filtration column equivalent to Trp. Formation of products was dependent on temperature, time, and species. Incubation of 125I-N-Tyr-DSIP and 125I-N-Tyr-P-DSIP, a phosphorylated analog, revealed slower degradation and, in contrast to DSIP, produced complex formation. An excess of unlabeled material did not displace the radioactivity supporting the assumption of non-specific binding/aggregation. It was concluded that the rapid disappearance of injected DSIP in blood was due to degradation, whereas complex formation together with slower degradation resulted in longer persistence of apparently intact analogs. Whether this could explain the sometimes stronger and more consistent effects of DSIP-analogs remains to be examined.