This is why medication is often so helpful. It has been demonstrated that therapy can change brain chemistry, as well. Research demonstrates that the combination of therapy and medication is more effective that either can be by itself. Sunlight affects levels of seratonin in the brain. In the winter when there is less available sunlight and we spend more time indoors, seratonin can be affected.
Some people become depressed only in the winter.
This is Seasonal Affective Disorder. Research has proven that depression is best treated with a combination of therapy and antidepressant medication. There are many different kinds of therapy. The one that seems to work best with depression is Cognitive therapy. This involves changing the way one thinks.