I have heard from more and more people that when the weather gets cold and grey, they don’t work out as much and also feel a little down. Their motivation to exercise and to get things done seems to dwindle. There are scientific studies that say that some people go into a depression from the shorter days, cold weather, and gloomy sky. 
Seasonal Affective Disorder (SAD) is still a mystery to scientists who study it. “Many things, including brain chemicals, ions in the air, and genetics seem to be involved. But researchers agree that people who suffer from the winter depression, SAD, have one thing in common, they’re particularly sensitive to light or the lack of it.”  Martin Downs, MHP, WebMD. Some studies are being done to look at people who suffer from SAD. Possibly some people have too much melatonin, issues with the time change and the shift in the circadian rhythm, interactions to serotonin and dopamine, and ions in the air. There is no question that people suffer from SAD. The issue is scientists have not yet determined how to treat it. 
I am thankful that I do not have this disorder, and I am certainly not an expert on SAD. For those who suffer from it, I would suggest you visit your physician to help find the best way for you to manage your disorder. 
For those who have some light complications, but they don’t affect your day to day actions, I would suggest exercising at least five days a week and getting outside as much as you can. I used to hate when I got up early in the morning and left for my corporate job when it was dark and also came back home in the dark. I would spend the entire workday in my office or meetings and never get any fresh air or sun on my skin. Do your best to get outside for a short time every day. On weekends get out for at least an hour each day. It could make you feel much better.
Staying busy is also helpful. Start taking a class during the winter season, or start working on a fun project. Keeping your mind occupied instead of zoning out in front of the television also helps improve your mood. 
Even if it is cold, get out of the house and spend time with friends. Get coffee, grab a glass of wine, go for a walk or snowshoe, anything to keep you feeling social and engaged with others. Staying cooped up in the house all the time can really get you down. 
When it is miserable outside, we like to play games. My favorites are shuffleboard (We happen to have a table at home, so obviously I love it!) Rummikub and Gin Rummy. Playing games always gets your mind off the gloomy weather and is good for a few laughs and a little smack talk when you are winning. 
Lastly, know you are not alone if your energy is low and you are feeling down during the winter season. If you have tried some of the suggestions I mentioned and they are not working, talk to your doctor. More than likely, he or she will have some tips that will help you feel better in no time.