SAD affects people in climates that are gray for a good part of the year. "Seasonal affective disorder is the PMS of the annual calendar," Dr. Northrup says. "What it is, is you're not getting enough serotonin. So the reason you're getting depressed is absolutely real. It's not enough serotonin. Natural light is a nutrient and it hits our retinas and it increases our serotonin in our blood so everybody in Chicago, everyone in the whole northern areas needs some natural light."
If you can't get enough natural light outside, there are other things you can do, the doctor says. "You need to get a good light box that has enough of the regular bright sunlight wavelengths. And those are widely available," she says. "Even, believe it or not, a lightbulb that's full spectrum lighting. Those fluorescent bulbs, and you just keep it out of the corner of your eye, where you can see it, reading at night, that will turn you right around."