REM rebound

REM rebound is what happens after you have been short of REM sleep: the following night contains proportionally more of it, arriving sooner and running longer than usual. It is one of the clearer pieces of evidence that sleep is regulated rather than merely permitted, since the system is evidently tracking what it missed and correcting for it.

The reason short nights hit REM hardest is structural. REM is not spread evenly across a night; it is concentrated in the final third, with the early cycles loaded with slow wave deep sleep instead. Cut two hours off the end of a night and you have removed mostly REM while leaving deep sleep largely intact. That is why the vivid dreams show up on the recovery night rather than during the deprivation.

What it says about repaying sleep debt

The practical implication is that sleep debt does not repay like a bank balance. There is an apparent order of priority, with deep sleep recovered before REM, so a single long night after a bad week restores some of what was lost and does not restore all of it in the proportions it went missing.

That is one reason the advice about sleep keeps returning to consistency rather than to catching up. A steady schedule avoids the debt; a weekend lie-in partially repays it while also pushing your body clock later, which makes the following week start worse. A steady schedule beats extra sleep makes that argument in full.

Alcohol and some medications suppress REM, which is why a heavy night is often followed by fragmented sleep and unusually intense dreaming. That is a well known pattern rather than medical advice, and anything that concerns you about your own sleep belongs with a doctor.