I've seen phrases like "set the alarm for after X hours of sleep" in many tutorials. But we can't really know WHEN exactly when fall asleep as we'd be unconscious by then and guessing how much time passed since we last looked at the time isn't really accurate. So how am I supposed to set an alarm? Am I right to assume that I should set an alarm X hours after the moment I got into bed and decided to fall asleep instead of when I fell asleep exactly(which isn't possible)?