MICHAEL TERMAN, PhD: They can say to themselves, I'm bad at this time every year, and I'm fine in the summer. They'll know, however, that it isn't some emotional connection with the holidays that's triggering their depression if it doesn't let up until April or May, because a holiday depression will pass shortly after New Year's.
ANNOUNCER: The cause of SAD is unknown, but research suggests where you live may make play a role.
MICHAEL TERMAN, PhD: SAD is widely prevalent throughout the population, and it's the worst the further north you go. In the middle tier of the United States, up until southern Canada, it's far more prevalent than in the south of the United States.
ANNOUNCER: Theories suggest the shorter days during the winter months throws a wrench in the works of the biological clock.
MICHAEL TERMAN, PhD: Our nervous system relies on a daily exposure to early morning light in order for the internal biological clock to stay synchronized with the external world. So when we allow our internal clock to drift later and later as sunrise drifts later and later and the days grow shorter, that's when we see the onset of all of these symptoms.
ANNOUNCER: Melatonin, a hormone involved in the sleep cycle has also been linked to winter depression.
MICHAEL TERMAN, PhD: Melatonin is a nighttime hormone. The nights are longer in the winter, and so our biological clock turns on melatonin earlier and turns it off later. Some people will start to sleep 12 or 13 or 14 hours.
ANNOUNCER: And similar to other forms of depression there may be an imbalance in the brain chemical that is involved in regulating mood.
