Winter Depression, also known as Seasonal Affective Disorder (SAD), is very common.
It results for about ten percent of all depression and since it runs in families a genetic basis is suspected.
Since it's most common in areas with short winter daylight, such as northern climates, short daylight seems to at least contribute to the problem.
If you are prone to winter depression, you probably will get it every year, starting in the fall and intensifying in the winter.
Unlike most other types of depression, symptoms include oversleeping and overeating instead of loss of sleep and little interest in food.
Other symptoms are in line with other forms of depression, including lethargy, loss of interest in everything, irritability and inability to concentrate.
Winter depression, not surprisingly, leaves in the spring.
However you may be out of shape and overweight and have severe strains on interpersonal relationships by then.
What can you do besides wait it out? The only approved medicine in the USA is Wellbutin (or its generic equivalent, bupropion).
Other antidepressants such as medications in the Prozac family may be effective as well.
Of course you can check with your doctor.
Besides medications, exercise and healthy diet, - basically a healthy lifestyle - can help.
Try to get some regular exercise, admittedly  more difficult in the winter, and eat well.
Light therapy has also proven effective, whether walks in the limited daylight, lots of artificial lights, or even a week on a tropical beach.
Even without any treatment, Winter depression is fortunately gone in the spring.