Seasonal affective disorder (SAD) is a form of depression that occurs during specific seasons, most commonly during the fall and winter. However, the condition can occur at any time of the year, including during the summer. The cause of SAD is not yet known, but it's believed to be related to the availability of sunlight. Light affects your internal body clock, which helps you regulate when to sleep and when to be awake.
Some scientists believe that a fluctuation in the body's production of melatonin, a hormone that helps induce sleep, might be the cause of SAD. Other researchers speculate that a lack of serotonin, a brain chemical or neurotransmitter that seems to be triggered by sunlight, is the cause of SAD. People who are depressed often have decreased levels of serotonin in their brains.
The incidence of the condition varies with geography. For example, it tends to be more common in the northern or polar regions. One study found a 10 percent occurrence in New Hampshire and only a 2 percent rate in Florida.
Common symptoms of seasonal affective disorder (SAD) include:
Seasonal affective disorder (SAD) can be difficult to diagnose because it's hard to differentiate from other forms of depression. Your doctor will ask about your symptoms and how long you have been experiencing them — in particular, whether you've experienced symptoms of SAD for at least two consecutive years and whether it was during the same season each year.
In addition, your doctor will want to know if the periods of depression were followed by seasons when you did not feel depressed. Lastly, your doctor will want to make sure that there are not other explanations for the changes in your mood or behavior.
If you are diagnosed with seasonal affective disorder (SAD), your doctor may choose one of several approaches to your treatment.
Reviewed by health care specialists at UCSF Medical Center.