Up with the lark!

Can anyone explain why on a weekday I struggle to open my eyes at 6:20am and long for a lie in but come Saturday morning I am up with the lark at 5pm after having 4 hours sleep? Perhaps it is my body clock. Last night I heard a bird singing at 11pm and thought, "What bird , in it's right mind would want to sing at that time of night when most birds are sitting in the dark, trying to be as still as possible, trying not to get noticed?" (Reminding me of those awkward planning meetings after school, in which nobody wanted to speak for fear of prolonging the meeting, clock watching until the meeting was over!) What to do today? I have already checked my emails (twice), had breakfast (porridge), put the washing machine on (just my own stuff)and fed the cat.