Apparently, I'm not exactly breaking new ground here, myself. I'm just adding a twist to it. Mine is a physical illness that causes mental issues. And if you can find a study that debunks my theory, I'd be happy to read it.

Here's just a few of the blurbs on a search of 'forward head posture depression':

The truth is, almost every single doctor who has created alternative treatment continues to ignore the real cause of depression: forward head posture.

Forward head posture is bad on the body because each time we get ... It leads to depression, anxiety, stress and all other negative terms that ...

The next time you're feeling sad and depressed, pay close attention to your posture. According to cognitive scientists, you'll likely be slumped over with your neck and shoulders curved forward and head looking down.