In winter in the Northern Hemisphere, we get less light. That affects our mood, and if it becomes bad, psychiatrists call it Seasonal Affective Disorder. I live in Denmark where our days are almost back to eight hours, but that still means the night is twice as long as the day.
You need to get as much light as possible. Some you get from being outdoors, but you might also be able to make more of the natural light you have available in your house or apartment. If you have blinds, pull them up during the day – even opened blinds block a lot of light. If you have window sills wide enough to sit in, make a point of sitting there and soak up some rays every day. Or you might be able to pull your favorite chair into an area of sunlight. It doesn’t have to stand in the middle of the floor all the time. But when you take a break, try to take it in natural light.