My sense of vocal cadence seems to get better at night, too; I get more into the flow of intra-line rhythm and pacing when I'm tired. (I also sometimes think about writing poetry as well, so it's a mixed blessing.) In fact, I tend to do a lot of my sustained efforts at work best when it's past 2am and I'm tired and want to go to sleep but have an even larger urge to finish up what I'm doing and just get it done.
My pet theory for why this is so: I can't relax, so I have to get tired to 'let go'. I quite probably have ADD; this would explain why I've felt slow and relaxed under the influence of Ritalin and other relatives of the ephedrine family. It's rather nice to have the hamsters in my head stop running around and around in their wheel and take a break and go sunbathing or the like. (Why I'd have a sun in my head for hamsters to bask in, I don't know. Sounds vaguely like something out of my dreams.
Sigh. I was going to rant about my social obliviousness, but I'm not making enough sense to write about that currently; oh well, it'll give me something to do at work tomorrow.