[Cerium]

I regularly wake up about 5 to 10 minutes before the alarm goes off. I think it has a lot to do with light levels. I tend to set my alarm to go off about 7:00 in the winter and earlier in the summer, so there is some light outside to wake up to.

[iamatworknow]

I'm also one to wake up 5 to 10 minutes before the alarm, but my alarm often varies. Some days it's 5:30 AM. Others it's 8 AM. So the ambient light level theory is out for me. But probably 6/7 days per week I find myself waking up a few minutes before it goes off.

[XCSme]

I also wake up sometimes 5 to 10 minutes before the alarm goes off and it's pitch black in my room, so probably by "internal clock" knows the time.