1. To lean or recline backward into some piece of furniture. She remembered that she hadn't taken the garbage out just as she was sinking back into bed. He anxiously sank back into his chair as the interviewer's questions became more personal.

2. To return to some previous cognitive state. I stared about the room nervously after being woken by the loud bang, but eventually sank back into sleep once it was clear that everything was OK. I can feel myself sinking back into depression again lately.