Depression and anxiety whilst lying down

[Bifford]

I've noticed that when I lie down to go to bed, I quickly begin to feel more anxious and pessimisstic about the world.  All the problems hovering over my head suddenly grow to terrible proportions.  When I get up and go out for a walk, however, I feel more or less normal.

I've heard some reasonings for this in terms of evolutionary psychology: when you're standing up, you're in a posture to fight or flee, whereas you're helpless while you're lying down.  I've also read science articles that say people take insults better when they're lying down.  Some psychiatrists still ask their patients to lie on a sofa during their sessions.  Is it possible the brain switches into another mode when you rest?