Potion Permit

You Pass out under one of Two Conditions. Either your Health reaches 0, or you are out too late [you will pass out at 2 am regardless of where you are]. It's rather difficult to just be killed early on without intentionally doing it but basic mobs can still kill you by just agroing and running into them.

It should also be noted that if you are waiting for anything timing or day related that Passing out will shorten your next day, as you'll wake up at Noon instead of 6 am.