Rise early. No really, keep your body on daylight savings time as long as possible whatever it takes (cutting the caffeine at 3, night caps, exercise) but get to bed early, and get up to catch that early morning light. Once you’re up bathe in it, go for walks, open all the curtains, you get the idea.
Then in the afternoon, while its still light out pull the curtains, and turn on all the lights. Cut off all contact with the outside world. (helps if your office is a remodeled closet.) Crank the music up, turn the screen brightness up, and try to ignore the outside world for as long as possible.
Doing this I can generally get a full days work in before the “I can’t believe its dark already” blues kick in, and sap my will to live.