Sleep debt is the cumulative shortfall between the sleep a person needs and the sleep actually obtained. If nightly need is eight hours and a person sleeps six, the debt grows by two hours per night. Over five weeknights that is ten hours, roughly a full night and a half owed by Friday.
The debt does not behave like a bank balance that can be cleared with one deposit. In controlled restriction studies, participants who sleep four to six hours per night show steadily worsening performance on a vigilance task across two weeks, while their self-rated sleepiness rises for a few days and then plateaus. People stop feeling worse even as they keep performing worse, which is why subjective sleepiness is an unreliable gauge of accumulated debt.
Recovery is partial and staged. A single long recovery night restores subjective alertness and reduces sleepiness substantially, and the brain shows a homeostatic rebound: more slow-wave sleep early in the night, as if the deepest stage is being prioritized. But measures that depend on sustained attention and on glucose handling do not fully return to baseline after one night, and after a week of restriction they can remain impaired after several recovery nights. The practical reading is that one night of catch-up helps, a weekend of long sleep helps more, but neither reliably erases a large accumulated debt, and the only dependable fix is not to build it in the first place.