We all know that regular, moderate exercise is good for us. But imagine what it would be like if all you did was exercise: if you ran, walked, jumped, or lifted 24 hours a day. After only a very short while, exercise actually wouldn't be that good for you because without rest and self-healing, exercise becomes counterproductive and even risky…and so it is with your mind. We spend all day (and sometimes all night, too!) in a whirlwind of thought. When there isn't something particular to think about (what to eat for breakfast, the tasks of the day, or what you're going to say in an upcoming meeting), we search restlessly for something to fill the gap-worries, hopes, television, and so on. We never allow our minds to rest. And without this precious self-healing time, our minds become exhausted and thoughts less trustworthy. Just as we need to stop moving our bodies every once in a while, we also need to stop moving our minds. But how? The idea can actually seem terrifying, not to mention impossible.
http://www.drweil.com/drw/u/ART02791/self-healing-meditation.html