It's the time of year when people start complaining about the winter blues. Up to 18 million Americans suffer from a serious form of depression called Seasonal Affective Disorder or SAD. Dr. Norman Rosenthal, psychiatrist and author of "Winter Blues," spoke to CNN about the condition.

CNN: What is SAD?

Rosenthal: SAD is a condition that occurs year after year when the days become short and dark.

CNN: How does it differ from the winter blues?

Rosenthal: Winter blues differs from SAD just in that it is milder. In this condition people will have a reduced quality of life, they will have less sparkle, less fun, less productivity, but they won't be impaired to the degree that they can't work or their relationships suffer.

CNN: What are the symptoms of SAD?

Rosenthal: People develop a very typical cluster of symptoms. Low energy, difficulty waking up, dragging yourself along, concentrating becomes a problem, work, relationships, all of these are difficult. Sleep increases, appetite increases especially for sweets and starch and people become sad, depressed, down. It really can become quite bad at its extreme.

CNN: What happens when the symptoms are extreme?

Rosenthal: In severe cases people with Seasonal Affective Disorder can be disabled, unable to work, unable to keep their commitments to other people, even suicidal. At its extreme it really needs medical attention.

CNN: What causes it?

Rosenthal: SAD results from three causes: genetic susceptibility, environmental factors like lack of light, and stress can bring it out if you are vulnerable. Lack of light is crucial. The light is providing us with certain chemicals in the brain and when we take that away in susceptible people the changes are not occurring and then you get the whole cascade of symptoms.

CNN: What are the treatment options?

Rosenthal: Get more light either by walking outside on a bright winter day or bringing more light into the home. This can be done with regular lamps, but if you have it badly you're better off getting a pre-made standard light fixture. Reduce stress, exercise, modify diets. If it's really bad, antidepressants can be helpful and cognitive behavior therapy can all be useful.

CNN: What if you don't do anything?

Rosenthal: You have to be active to combat SAD. You can't just be passive, otherwise it engulfs you and you're like a hibernating bear all winter long.