Depression is the "common cold" of psychological disorders.  The World Health Organization considers depression a high priority concern and most everyone has been depressed or knows someone who has been depressed at some time in life.  Its symptoms include sleep disturbances, loss of appetite or overeating, and anhedonia (inability to experience pleasure).  While it comes in different forms and intensities, some of its characteristics are enduring.  Depressed people think in different ways from their non-depressed counterparts.  Their neuro-chemistry is different.  They have low self esteem.  They are inactive.