During school time I can't but use an alarm. No way in hell I would get all the rest I need by, say, 6 a.m. if I have 8 a.m. class and such. During summer, I do not use an alarm unless I have some appointment of some sort. Usually I plan my outings during the afternoon or at night so I'm fine. But it's summer, let me get all the sleep I can get before Uni starts.
It's only logical that whenever you wake up on your own, you'd feel less tired, since you waking up = your brain telling you it's rested all that it can.
Edit: Sometimes I do have an internal alarm clock, but it's very unreliable.

Last Friday I had a doctor's appointment at 8 a.m. and I actually woke up at 7 a.m. and realized I should get ready (even when I slept at 3 a.m. that night) but i fell back asleep.

The alarm went off a few minutes after I dosed off so it was fine.
I don't trust myself anyway, I always have an alarm scheduled at, for instance, 8:00 and 8:05 and 8:10 and 8:15, just in case I dismiss the alarm instead of snoozing it. During exams, I schedule it at 8:01 and 8:02 and 8:03 and so on, till like 8:30 (

) because I would be SO sleep deprived I fall directly back asleep and say "**** it I don't care" when i actually do care to study but i'd wake up feeling EXHAUSTED.