For the past couple of years, I've tried to write, or prepare for writing, for four or five hours a day, five days a week. (Weekends off). As such, waking up and stumbling to the computer becomes such a habit that I don't really think about it much, and the writing usually comes fairly easily.
However, this gets disrupted when I take time off. Last week I didn't write on Thursday or Friday due to going to a professional conference . The conference was enriching--I learned a lot and got some good ideas for future research, but this morning, when I got back to the computer, I noticed that it didn't feel as natural. The writing felt forced and more stressful than usual. If past experience holds, I'll be back in the groove by tomorrow, but it's a good reminder of how useful a writing routine is.