Seasonal affective disorder (SAD) is a type of depression that's related to changes in seasons — lack of sunlight.

SAD begins and ends at about the same times every year. If you're like most people with SAD, your symptoms start in the fall and continue into the winter months, sapping your energy and making you feel moody.

Seasonal affective disorder is a subtype of major depression that comes and goes based on seasons. So symptoms of major depression may be part of SAD, such as:

Feeling depressed most of the day, nearly every day

Feeling hopeless or worthless

Having low energy

Losing interest in activities you once enjoyed

Having problems with sleeping

Experiencing changes in your appetite or weight

Feeling sluggish or agitated

Having difficulty concentrating

Having frequent thoughts of death or suicide

The specific cause of seasonal affective disorder remains unknown.

Your biological clock (circadian rhythm).The reduced level of sunlight in fall and winter may cause winter-onset SAD. This decrease in sunlight can disrupt your body's internal clock and lead to feelings of depression.

Serotonin levels. A drop in serotonin, a brain chemical (neurotransmitter) that affects mood, play a role in SAD. Reduced sunlight can cause a drop in serotonin that may trigger depression.

Melatonin levels. The change in season can disrupt the balance of the body's level of melatonin, which plays a role in sleep patterns and mood.

Factors that may increase your risk of seasonal affective disorder include:

Being female. SAD is diagnosed more often in women than in men, but men may have more-severe symptoms.

Age. Young people have a higher risk of winter SAD, and winter SAD is less likely to occur in older adults.

Family history. People with SAD may be more likely to have blood relatives with SAD or another form of depression.

Living far from the equator. SAD appears to be more common among people who live far north or south of the equator. This may be due to decreased sunlight during the winter and longer days during the summer months.

Treatment for SAD may include light therapy (phototherapy), psychotherapy and medication.