Oh I HATE when that happens...used to do it all the time when I was driving over the road.
Worse feeling, is forgetting where you are, where you're going, and nothing looks familiar.
I had to literally wait for a road sign to figure out where I was.
I guess our minds sometimes go into such a deep sleep that it does not send messages to the brain of what time it is. I think there is also a message in there that, you need more sleep.
Anyway, now that that wierd stuff is over I'm off to the Pub for a few brews. And I don't have to be at work for another 11 hours. LOL.