The Paradox of Time
Does this ever happen to you: It's Friday, and you look ahead to Saturday or Sunday and see a large block of time that you haven't designated for anything, and you think "I can fill that with some really quality writing time or research or working out at the gym!"
You look forward to it, you may even arrange your workspace or gather your gym clothes the night before. The weekend comes, and starting at 2pm (if that's the time you designated), you suddenly find something else you need to do. But you feel guilty.
Still, the guilt doesn't move you to your work space or the weights. The spark that was present those nights before just isn't there. Let's say you finally open up the file, look at the words glowing in front of you, or make it to the gym, but your body feels like the gun's gone off in the 100 yard dash, Literally. And you jump up out of your chair and head for the fridge or say "20 minutes on the elliptical machine should do it for today!"
One of my brothers described this very sort of thing when he had his first kid (my first nephew.) "I think of all these great things I can do, I can get done, once he's down for his nap," my brother said. "But when he's finally out, all I want to do is sit in a chair and turn into broccoli."