Yes, I believe when the body begins the process to sleep, the mind floods the body with a chemical to shut you down for sleep. I guess they can easy test for that.
Hormone called Melatonin.
Iirc that's released before sleep to make you tired (That would be present when people are still awake but tired) it's the chemical it acts against is the one I mean, the one that keeps us awake during the day. Can't find its name at the moment .
I think we have a forensic guy on here hopefully they'll drop in and say what's what.