SAD (seasonal affective disorder), commonly known as winter depression, is an emotional or emotional disorder that usually occurs at the end of the season and winter. Most SAD patients are in good health for most of the year, but they feel depressed during the season when the day becomes shorter and the night becomes longer. In the tropics, SAD is rare even if it exists. However, it exists significantly north of 30 degrees north latitude or south of 30 degrees south latitude. Winter depression is very common among Scandinavians. In the sixth century AD, Gothic scholars first described SAD in their writings. Now science believes that seasonal mood swings have a lot to do with sunlight.