Psychologists have observed that learning is somewhat state-dependent: that is, if you learn something in a certain physical or mental condition, you will recall that learning better in that same state. If you always practice piano at the same time in the same place, just being in that place at that time will put you in the proper frame of mind to play. And, conversely, you'll have a harder time playing when you play at a radically different setting.
My writing, alas, is really state-dependant now. I find it easy to write at the beginning of the day, and torturous to write at night. I had cut out time in the morning to write because it was the least susceptible to disruption by my work schedule . . . but no time is completely safe from disruption. I was at UNC's FallFest last night and didn't get in until 1:30 am . . . which was a blast, but it's waaaay past my bedtime. And then I had to go to Winston-Salem in the morning to be with my mother-in-law for a doctor's appointment. All it takes is two back-to-back disruptions to smash out my writing time . . . and if you string enough of those together, a healthy habit is toast in no time.
I've always taken great pride in being flexible in my schedule, and willing to work long and hard . . . but I am continually rediscovering that "long and hard" is not nearly as efficient and "conscious and consistent."