How far into your PhD are you? Haven't you been writing your thesis all along?
I'm not trying to be rude, I'm just interested in how the PhD process is different for different disciplines. I'm in the social sciences (education) and I started writing my literature review very early on in the process. Since the literature review (albeit with some changes along the way) will form part of the thesis, I can say that I started writing my thesis over a year ago. It's impossible to really pinpoint a particular point in time when the process started. I started writing my methodology chapter as I went along, and now that I have almost completed data collection I am spending longer on the writing process. I anticipate an intensive writing-up period later this year.
Anyway, I guess because the process is so drawn out, the nerves are not so focused on the writing part. How do other disciplines do it?