This article from the American Craft Council points to some academic research that shows the repetitive motions of knitting relax the part of our brain that control the flight or fight response. Other craft activities have been shown to light up the pleasure center. More simply, the act of making something can feel like progress in a time when you feel adrift.
So, if you are feeling down, it might not be the time to attempt a sweater if you are a new to knitting. Start small with something fun! Pick something that is in your crafty wheelhouse, or a friend can show you how to do.