The specific cause of seasonal affective disorder remains unknown. Some factors that may come into play include:
- Your biological clock (circadian rhythm). The reduced level of sunlight in fall and winter may cause winter-onset SAD. This decrease in sunlight may 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, might play a role in SAD. Reduced sunlight can cause a drop in serotonin that may trigger depression.
- Melatonin levels. Darkness increases production of melatonin, which regulates sleep. As winter days become shorter, melatonin production increases, leaving people with SAD to feel sleepier
- Less production of Vitamin D. Vitamin D is believed to play a role in serotonin activity. Vitamin D insufficiency may be associated with clinically significant depression symptoms.
Seasonal affective disorder is diagnosed more often in women than in men. And SAD occurs more frequently in younger adults than in older adults.
Factors that may increase your risk of seasonal affective disorder include:
- Being female. SAD is diagnosed four times more often in women than men.
- Family history. People with SAD may be more likely to have blood relatives with SAD or another form of depression.
- Having major depression or bipolar disorder. Symptoms of depression may worsen seasonally if you have one of these conditions.
- 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.