Cecil answers your question, Isabelle.
Mine does this, too!!! For an hour usually… Wouldn’t bother me that much if it wasn’t the hour before I have to get up. She’s started doing this in the last month or so, I guess I’ll have to have a roomie to roomie talk. God, I hate those.
And I think I’d rather sleep soundly til the last minute I have to get up. Why bother waking up and stirring fretfully, hitting the snooze button all the time?
(That and I tend to just switch off the alarmclock by accident and oversleep.)
I’m with Fretful Porpentine. I just don’t see the point. Once I’m awake, I’m awake, and even if I’m out of it there’s no way I can go back to sleep. I rarely sleep through to the alarm, when I use it; I usually wake up about thirty minutes before it’s supposed to go off. I guess my brain knows I’ll be getting up but doesn’t like to be startled awake.
I also can’t function on less then about eight hours of sleep for more than a couple of nights in a row. It does make getting up easier when I’ve had ENOUGH SLEEP. Maybe some of you ought to try it? 
I know that if I train myself not to hit the snooze button, I’ll simply sleep through the alarm. I’ve slept through alarms that were blaring right next to my head for two hours (it stops after 2 hrs), multiple times.
I got the problem licked.
I work midnight shift so I get home at 8:30, eat, watch a little TV then bed about 10. I wake up whenever I feel like it, usually about 4 or 5 and then have my whole day.
I havn’t used an alarm clock in 3 years.