I have one: I know I need my private/down time and I've learned to honor that by closing my office door during my lunch brek, saying no to things that rob all my free time, and I arrange "dates" with myself. Sometimes they are "cry dates; "sometimes they are "celebration dates," and sometimes I only seek some quiet time. Society, and my job, reward and expect us to be social and out-going, and it feels good to accept and honor the fact that that's just not me.