I asked a doctor this once, a long time ago, and hopefully can mash together enough of their explanation to give you a vague start. When we sleep our brains are still very busy. Our chemistry changes too. There are different kinds of sleep, and different phases. Beyond that, things like dreams (which happen during a certain phase) can stimulate chemistry on their own (terrifying dream = cortisol or adrenaline in our bodies).
So, the answer is complex. Depends on what phases of sleep you had, what dreams you had, your specific cocktail of neurochemistry before, during, and after.
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