hmm a mixture of things
firstly, because when I wake up, it's very rare I can go back to sleep so would prefer to be active than just lie their wasting time.
secondly, my hatred for sundays. when I was a kid, the fact that everything was slow on a sunday used to drive me nuts- even down to the shops opening at 11 A.M and not the standard 9 or 10 on the other days
also: it's a case of the longer you lie in bed, the more day gets wasted and I don't want that- I want to make the most of the day rather than let it slip by while I'm under the covers