Why does the sleep need not accumulate proportionately from previous days where we didn’t sleep enough?

Question

If you slept only 4 hours the previous day and you got a good night’s rest the next day, you don’t wake up 12 hours later but rather about 8-9.

In: Biology

Answer 1

You absolutely do accumulate fatigue if you do not sleep enough over a number of days. I can’t understand why you would not know this.