Depression is one of the symptoms of PTSD.  I realized after several years of counseling I use depression to put a damper on the anger generated by the hurt, fear and frustration I experience every night in my nightmares.  If I am depressed and tired enough, I won’t have nightmares.  A break of sorts.  I am learning to accept the depression for the in-between step as I learn to mange and feel my emotions.  The interesting thing about depression it is like running a fever.  When your emotions are hurting, frustrated or scared, anger jumps in and depression softens the anger.  I am noticing that as I take a healthier approach to my emotions, I use depression less often.  I have a new appreciation of what depression can do for me.