"In fact, the brain starts working less and less," says Duhigg. "The brain can almost completely shut down..
And this is a real advantage, because it means you have all of this mental activity you can devote to something else."
That's why it's easy — while driving or parallel parking, let's say — to completely focus on something else:
like the radio, or a conversation you're having.
"You can do these complex behaviors without being mentally aware of it at all," he says. "And that's because of
the capacity of our basal ganglia: to take a behavior and turn it into an automatic routine."
http://www.npr.org/2012/03/05/147192599/habits-how-they-form-and-how-to-break-them