Get out into the sunlight for 10 minutes a day.
Eat one serving of protein and a multivitamin.
Clamp down on your anger and your despair, they will still be there in January to fuel your work. In the meantime don’t let them destroy you.
Call your mother and your siblings. It’s Christmas.
Vastly reduce your expectations. For me, I am thrilled that I can now solve a puzzle while looking at the answer; a couple of days ago I couldn’t even do that.
Be very open about how sick/different/disabled you are in December. When you make a mistake or can’t do something, explain: “It’s December, and I’m not thinking clearly because of seasonal depression.”
Eat rather than drink.
Hang on with your fingernails til the 27th, that’s the day that I can FEEL there is more light. ymmv