I'm programmed to wake up at about 6am or 6:30am automatically. However, if I'm on Morning shift, the sleep is usually not so deep and I have to wake up at about 5am to 5:30am.
Even on days off, I'll still automatically wake up at about 6:30am, then have to walk around a bit, or just lie in bed eyes open for a while, then return to sleep.
When I'm on Night duty and have to sleep in the daytime, I'll definitely wake up at 2pm, and then again at 4pm. Don't know whatever for!
That's why sometimes, I have to go to the gym to tire myself out so I can sleep right through.
Anyways, when I'm on nights, I usually survive on 2 hrs' sleep on average.
Hard to get rid of the internal alarm clock.
With shift work, it's even more messed up now.
Used to be a Morning Lark but looks like now, I've mutated to become a Night Owl!