Writing novels is hard, and requires vast, unbroken slabs of time. Four quiet hours is a resource that I can put to good use. Two slabs of time, each two hours long, might add up to the same four hours, but are not nearly as productive as an unbroken four. If I know that I am going to be interrupted, I can’t concentrate, and if I suspect that I might be interrupted, I can’t do anything at all. Likewise, several consecutive days with four-hour time-slabs in them give me a stretch of time in which I can write a decent book chapter, but the same number of hours spread out across a few weeks, with interruptions in between them, are nearly useless.
Another factor in this choice is that writing fiction every day seems to be an essential component in my sustaining good mental health. If I get blocked from writing fiction, I rapidly become depressed, and extremely unpleasant to be around. As long as I keep writing it, though, I am fit to be around other people. So all of the incentives point in the direction of devoting all available hours to fiction writing.

Source: Neal Stephenson – Why I Am a Bad Correspondent