Some people experience a serious mood change during the winter months, when there is less natural sunlight. This condition is called seasonal affective disorder, or SAD. SAD is a type of depression. It usually lifts during spring and summer.
Not everyone with SAD has the same symptoms. They include
SAD may be effectively treated with light therapy. But nearly half of people with SAD do not respond to light therapy alone. Antidepressant medicines and talk therapy can reduce SAD symptoms, either alone or combined with light therapy.
NIH: National Institute of Mental Health