Some people who think they have the holiday blues may actually have seasonal affective disorder (SAD), an expert suggests.

"SAD usually occurs in those who already are diagnosed or afflicted with a type of depression. It occurs with the change of the seasons, beginning in the fall and staying with you throughout the cold, dark winter months," Dr. Jason Hershberger, chair of psychiatry at Brookdale Hospital and Medical Center in New York City, said in a hospital news release.

"Your energy drops and your mood will swing. Many shrug off the depressed feeling as the winter doldrums, denying that they may be suffering from mild to severe seasonal depression," he explained.

Lack of sunlight is the main cause of SAD, but age, sex, where you live, and your family history also affect risk, researchers have found.

"If you are a young woman living far from the equator with a family history of SAD or a previous diagnosis of clinical depression or bipolar disorder, you are most at risk," Hershberger said in the news release. "Women are found to have more severe symptoms, while young people are often at higher risk than older adults," he added.

"Living far from the equator means there is less sunlight, especially during the winter. Any family or personal history of depression, bipolar disorder or SAD makes the disorder hereditary," Hershberger explained.

If you think you have SAD, seek medical help or talk to a family member or friend, especially if you have any thoughts of suicide, he advised.

"Before you see your doctor, make a list of your symptoms; recent life changes like divorce, work or school; medications and supplements -- including vitamins; notes on when your depression started or became worse; and a list of questions you have for your doctor," Hershberger suggested.

People suspected of having SAD will undergo a physical and mental examination, followed by blood tests to check for any other possible causes of symptoms, he said.

Doctors will usually recommend phototherapy, which is "a form of light therapy to help your brain produce the chemicals, like serotonin, you need to feel healthier and happier," Hershberger said. Light boxes are widely available for less than $100 to about $300.

Antidepressant medication may also be prescribed along with so-called "talk therapy" to help you identify negative thoughts, and learn how to cope with your stress and anxiety, he added.