Human mind is one of the most complex things ever discovered. All the process going in there is beyond the grasp all the major league scientists. So the truth is we can't be sure.
Then again, thinking about it, we know that most of the population wants to be with the company of people. Very few wants to be left out and be alone. But even for the most party animals, being left alone sometimes feels good.
This is because, it is the time you will have to time to go over all the things. It's the time your mind summarize all that was going til then. The time for future planing. The moment when you are lost in your thoughts. The time when your mind decides to go exploring all the areas it hasnt yet been to.
That time you are in a state of meditation. Your body and soul are at peace.
So just like yoga refreshes you, this also gives a positive response to your system.