Some individuals experience serious depression when the seasons change and the days are shorter and darker. Some experts believe that reduced levels of sunlight are responsible for causing these changes in mood. Indeed, light treatment has shown to help reduce the symptoms of depression. Therapy can also help people with seasonal affective disorder (SAD). Here is a good article on Seasonal Affective Disorder (or SAD) where you can find more information.
https://www.nlm.nih.gov/medlineplus/seasonalaffectivedisorder.html
