These are the kinds of days when I wish I didn't need to sleep.
Now, most days, I really do enjoy my sleep. I find a certain brand of pleasure in settling in at the end of the day (or so) and sinking gradually (or gratefully) into unconsciousness. And I recognize that sleep is an important part of my/any creative process. The opportunity to let the mind rejuvenate and create mental connections between disparate ideas is one that I am truly grateful for.
But when I'm in the throes of a creative project, I really don't want to be bothered by the limitations of my mortal body. It's a time like this -- when I'm excited about and focused on a project to the degree that I have more to say about it than I'm letting on, even to myself -- that I would much rather stay up through the night in order to pound out another two thousand words, or another seven plot points, or three more details about the worldbuilding, than turn in for the night and get some rest.
There are some nights when I'm more energized in the face of my work than I've been all day. I couldn't possibly think about going upstairs, brushing my teeth, and settling in for the night. Come on, I'm just getting started!