sink down
1. To fall or lower (oneself), as due to exhaustion, weakness, despair, etc. I came in the door and sank down onto the sofa, totally wiped from such a long day at work. I sank down to the floor and cried when I heard the news that Michael had died.
2. To move or appear to move to a lower position or location. We sat watching the stars begin to appear as the sun sank down in the sky. I sank down in my chair, trying to tune out the sound of my parents arguing.