Sleep/restfulness is a very fickle thing. It's true that tiredness does stack. It is also true that you can't really "make up" for lost sleep. Of course it is good to get more sleep the night after you get poor sleep, but you aren't really making up for the lost sleep, you are just getting your body back to its original baseline. Also, if you sleep too much it's easy to feel groggy. I remember one summer when I fell asleep during Conan O'Brien (which for me was from 11:30-12:30) and woke up at noon every day and I was still really tired. Too much sleep isn't good, either.
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