ALL WAYS thinking
It's funny you say that, thinking is an automatic function to all humans that you can't really shut off. That's why you get people doing all kinds of drugs or activities to "get their mind off of things" as to de-stress.
A persons level of intelligence, and exposure to various concepts and subjects though can for the most part either ease this automatic process, or make life a living hell.
The more intelligent one is, the more unhappy they generally are.