Start at 10.
I know many writers and they all have different techniques. If you are ever lucky enough to have the time to write then please use it wisely. So many people I've known get up at 7.30 on Day One, hit the kitchen table at 8, pencil sharpened and ready to rock and roll, write like a demon for two hours and then run out of steam. By Day three they're down the pub and the whole project has hit the skids.
Writing a book is a long and painful process. The best tip I've ever had came from my friend Rob Rankin - and he churns out books like there's no tomorrow. He told me to start at 10. He said that if you set yourself an earlier starting time all you'd do is miss it - and then you wouldn't start all day. So now I start at 10 - and it works!