For me, at least, it was in that sweet spot of cognitive demand that allows for deep reverie, but is demanding enough that I didn't become bored with just thinking.

Personally, I find I can't slip into deep thought while just sitting on the couch, I need some kind of other stimulation to meet my optimal level of arousal. When I really need to think about something, I always wind up pacing, cleaning, running errands, playing minesweeper, etc.

Of course, this is after you get used to the job, which takes several days to a few weeks.