Well, the idea is that you'll be tired enough that you'll almost always pay it back pretty promptly. So your body doesn't have to remember for a really long tim

Well, the idea is that you'll be tired enough that you'll almost always pay it back pretty promptly. So your body doesn't have to remember for a really long time because you don't keep a balance for that long. For example, after you turned in the papers/took the exams from the all-nighters in college, you probably crashed and took a long nap or slept 12 hours that night or something. It's not exact, obviously. I have found that you kinda get a "discount" when you get enough into sleep-debt. I slept about 15 hours over 6 nights once, and after basically sleeping for 24 hours straight, I was totally fine. So that was only like 16 hours of extra sleep (because 8 of the 24 have to count towards that night), which means with that the initial sleep + the paid back sleep was only ~31 hours for 6 nights, which is still only 5 hrs/night.