Biological alarm-function

I often wake up just before the alarm starts on my cellphone.

After hours of sleep I can wake up less than a minute before the alarm goes off.

You might argue that its simply me being used to get up at a certain time,

but that will not hold.

I always keep changing the set time for getting up.

If I need to be somewhere early in the day,

I might set the time to 07.03.

This is to have the psychological impact that there is no time to snooze,

and that feet must hit the floor immediately. 

Other days the urge to get up&about is of less importance,

and I might set the alarm to 06.53 to achieve the opposite effect,

that of having time to spare and allowing for a mellow start of the day.

 

Matters not what time of day,

I pretty much wake up a minute or two before the alarm goes off.

Any body else familiar with this?