New here — in plain terms, what's the difference between DSIP and Selank?

Not interchangeable at all, good instinct. In broad strokes: DSIP is the one historically tied to sleep / delta-wave research, going back to 1970s animal work. Selank is a synthetic peptide developed from a fragment of a naturally occurring immune peptide and is studied mainly as an anxiolytic — the sleep angle is more indirect, via the anxiety/stress side.

So they land in the same category because sleep and stress overlap, but they're studied for pretty different primary reasons. Both have limited human evidence, which is the recurring theme around here.

Good answer above. For a starting point, the pinned resource thread has the primary literature with notes on how old and how small each study is — that context matters a lot with these two.

And the standard reminder since you're new: everything here is educational, people's experiences are personal anecdotes, and for any actual sleep or anxiety issues a qualified clinician is the right person to talk to, not a forum.

This is exactly the conceptual split I was missing — thank you both. Heading to the resource thread now. Nice to land in a place that answers the question without pushing anything.