As someone who regularly has trouble with his sleeping pattern and ability to perform that task at all. I can confirm that I've woken many a time not understanding the time, day, date, what I was meant to be doing, or when I actually fell asleep. I'll remember stuff from earlier in the evening but after a certain point it's just black. I know I was up later because I'll have had a full blown convo with someone online. Or I'll be a hundred pages deeper in to a book I'm reading but I'll have no recollection of the events themselves.

I believe it has to do with your body clock, sleep requirements and how deprived you've been prior to the sleep. It's not a nice feeling and it's hard to explain to your office boss. That being said, my Doctor last time I mentioned it said it was fairly routine but didn't explain it at all. I'm not settled by that nugget of information one iota.