That's really bizarre. Firstly, it's happened before, even when I haven't had my alarm set. Secondly, it's Monday morning, so it's not as if my body clock is in the habit of waking up at this exact time. Normally I set my alarm for 6:20 and it wasn't set over the weekend.
I googled and found this article which might explain it:
(Jan Born appears to be legit.)
I'm almost tempted to phone Dr Karl this Thursday to ask the question.