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I can't speak for OP, but some people sleep through alarms if they haven't slept enough. My oldest will legit just sleep through his phone alarm if it has been less than 5 hours since he went to bed. I've just heard his alarm go off in his room for 20 minutes straight before I walk in there and shake him awake. Shit is a problem.
I am assuming he won't be able to go to bed at a decent time. I wake up at 6:30am most days and I know I couldn't go to bed at 6:30pm. If he woke up at 3pm, it would be a tall order to fall asleep before ~4am I'd figure. Which gives him ~3 hours of sleep.