Shorter, darker days can cause winter depression also known as seasonal affective disorder or SAD. The season runs from about November through early spring. Symptons include fatigue, weight gain and a weakened immune system. About 10 million Americans suffer from SAD. So what causes this? Lack of light during waking hours from November until early spring is the culprit. Lack of light interrupts the body's natural rhythm and endorphin production. SAD is more likely to effect women ages 18 to 40. People who live in areas with dramatic climate change are more vulnerable. So what to do? A Swiss study found that being outside in winter sunlight for an hour dramatically improved sufferers moods. Try taking a daily walk or get outside and play with the kids.