seasonal affective disorder is more common in regions of the world that are farther away from the equator,and so experience great seasonal changes in daylight hours. In Northern Europe, an estimated 12 million people suffer from SAD. In the United States, approximately 4% to 6% of the population may have it and a large number have milder "winter blues." This condition strikes mostly women in their 20s to 40s. scientists aren't sure why some people get seasonal affective disorder and others don't one theory is that some sufferers may have a biological predisposition to it.