I’ll use the simplest analogy I can, and rephrase your question:
“Why can a glass of water that is half full still leave is thirsty, but it cannot contain water over the rim for when I’m really thirsty?”
It’s a matter of… At some point, you’ve hit the maximum amount of water you can put in the glass… And likewise, after some amount of sleep, you have reached about the maximum amount of energy your body can store.
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