I've repeatedly heard it said by sleep experts that one cannot "catch up" on missed sleep. The best explanation of that statement I've heard so far is that a lack of sleep has physical effects on one's health, and those accumulate and cannot be reversed. Paraphrased and simplified, each missed hour of sleep shaves off an hour of your life, or something to that effect.
: Most recently in some interview with Matthew Walker, but I've heard other people state similar things.
Apparently it doesn't work to sleep 6 hours during the week and 13 hours during the weekend; that won't equal an average of 8 hours per night for your body and will have negative longterm effects.
Clearly though, it must be possible to "catch up" on sleep in some sense. If I don't sleep one night I will feel like crap the next day; but clearly after sleeping well for a few nights after that I will feel fine again, so my body must have returned to some normal equilibrium.
So, what exactly is meant by the statement that one can't catch up on sleep? Taken as an absolute statement it would mean you'd accumulate a lot of missing sleep time over the course of your life, and I don't really feel like I've missed several weeks or months worth of sleep right now; so this statement must require some qualification.