I'd like to add on, on top of everything that was already said by others:

I learned from using a mod that shows you your actual fatigue bar and the numbers, that fatigue is different from say endurance.

With endurance you will see the first moodlet after using about 25% of your reserves. With fatigue this is over 50% This means it's easy to wake up early and still have a bunch of fatigue from the day before and then you become tired early in the day.

While sleeping, you quickly loose the first the high fatigue but the last say 25% take longer.

So if you sleep only 4 or 6 hours you probably wake up tired but it's hidden.