If you find that your mood starts to drop during the Winter months, you may be suffering from Seasonal Affective Disorder (SAD).
Often called Winter depression, SAD is a mood disorder brought on by the lack of light due to shorter days. The symptoms of SAD can be quite severe and are in line with major depression, but typically disappear with the arrival of Spring.
If you suffer from SAD, some symptoms you may experience are irritability, a drop in energy level, and weight gain. Treatment includes light therapy, monitored doses of melatonin, or antidepressants.
I definitely get a little bummed when the days get shorter during the Winter, but do not suffer from SAD. I do try and spend as much time as I can outdoors before the sun sets though. How about you?