Full on Seasonal Affective Disorder here, been part of my life for at least ~25 years. Everything is usually hard work from September to March, on days when the mental fatigue is really bad, you want to hide away from the world but the best thing you can do is get outdoors for ~60mins+.
Light boxes can help with the extreme fatigue, but they don't cure anything like all SAD symptoms, such as depression and carbohydrate craving.
A majority of my work hours are outdoors, which helps, but frequent recreational cycling for the last two years has really helped my SAD and fitness (not to mention help me lose 15-20Kg). From a very basic ~30 miles of cycle commuting each week in 2016, I've done ~5200 miles and ~365000 feet of climbing so far this year, hoping to reach the 400000 feet milestone before the year ends.