This happens to me also, Sanders, even when I used to work second and third shifts. Nothing circadian about it, IMO. Still happens now, the only exception being if I’ve been drinking (way) too much the night before, but those days are few and far between for me now. I’ve never understood it, so I can’t help with an answer for you, but it appears (from the comments so far) that there are quite a few folks with the same ‘condition’. I’ve mostly found it useful rather than annoying.
My dad told me years ago that he’s the same way. Even when he was in the army, he’d wake up a minute or so before the rest of his barracks, even if he was dog-tired the day before. No clocks. He did mention this as being useful (at the time), because he got a jump on rousing from sleep before being forcibly woken up.
Oh–also, I really suck at telling time during the day as well. Must be a package deal…