True but considering he doesn't know how much time he has to find Sarah before the Terminator does you'd think he'd be a bit more panicked at having fallen asleep.I never took it as a nap. Imagine yourself being born INTO the apocalypse. Flying and stalking death everywhere.
How well did you sleep? Then you get to another time and everything's silent. Your body just shuts down, gives you what you need.
Imagine a very exhausting day...times a hundred.