I do writing marathons. My work has been such for some years now, that I have to work around the clock for months with very little down time, so when ever I get a vacation I just sit down and write. On these holidays I never follow any reasonable bio rhythm either; I sleep when I feel like it and exercise whenever I need a break from writing.
In the beginning I rewrote almost everything every time I started another writing phase, I just felt I couldn't get into the mood without doing it and the old text looked all wrong. Looking back on it, I can surely say that it is the worst way of writing. By far. I've been writing my first novel for several years like that, and when it was close to being ready, I found that I needed to rewrite the whole thing anyway. Would've just been smarter to get the first draft as quickly as possible and rewrite only when it was completely ready. If I'd done the process with a little bit more brain and a little bit less all-over-the-place, I would be writing my third book now. But I've learned that lesson now and it's gonna be a lot easier to go onto future projects from now on. As we say back home, I've paid my school fees and now I'm smarter.
If you don't get to write all the time and you feel difficult to refocus on a certain style or voice, don't do what I did. Just write the darn thing and go back to fix style and voice when you're done with your first draft.
I occasionally teach motivation (in a very different context), and the only thing I can say for sure is that no one does any one thing for any one reason. Sometimes I slap myself into writing by calling myself lazy, sometimes I just can't wait to write. Most times though, I just sit in front of the computer and have the text document open. Even if I fool around on message boards, the project eventually sucks me in and before I know it, I'm writing and it's five hours later. In the back of my mind, however, there is always that one little elf, that keeps saying that this is the profession you should be pursuing, and at some point you'll see a rainbow of crap at the end of which there is a broken chamber pot with a few shillings in it.