Seasonal affective disorder, or SAD, is a form of depression that primarily affects people during the fall and winter months when there is less daylight, particularly in locations farther from the equator. This lack of light can disturb the internal clock and may lead to feelings of depression. The change in seasons can also influence the body’s melatonin and serotonin — natural substances that play a role in sleep timing and mood. When combined, these factors may lead to SAD.