When you're on a regular schedule, your body gets in a rhythm where you'll usually end up waking up at the exact same minute every day, even without an alarm.
When you don't maintain a regular schedule (or you're in sleep debt), your body can't do that. So your alarm may start buzzing in the middle of "deep" sleep, and you get stuck with sleep inertia.
Smart alarms like this try to compromise by waking you up in "light" sleep, or ideally in moments of wakefulness that you'd otherwise pass up and go back to sleep.
When you don't maintain a regular schedule (or you're in sleep debt), your body can't do that. So your alarm may start buzzing in the middle of "deep" sleep, and you get stuck with sleep inertia.
Smart alarms like this try to compromise by waking you up in "light" sleep, or ideally in moments of wakefulness that you'd otherwise pass up and go back to sleep.
(If you're not familiar with the concept of sleep cycles: https://secure.wikimedia.org/wikipedia/en/wiki/Sleep#Sleep_s...)