Feeling sad or lethargic now that the days are shorter and the temperatures have dropped? Sleeping more often than you want?

You may be suffering from SAD (Seasonal Affective Disorder), which is not some pop-psych mumbo-jumbo, but rather a very real type of depression that normally occurs in winter months as a response to the brutal changes in the natural day-night cycle. And it's widespread: SAD is estimated to affect 10 million Americans, according to Psychology Today.

RELATED: 24 Steps to a New, Healthier You

Symptoms include thoughts of suicide, irritability, difficulty concentrating, fatigue, feelings of hopelessness, weight gain and more. It's brought on when the reduced levels of sunlight mess with your internal clock, and your serotonin (the chemical that affects mood) and melatonin (the chemical that plays a role in your sleep patterns) levels get thrown out of whack.

RELATED: The Only 5 Exercises You'll Ever Need

Some naysayers scoff at the idea of the condition (partly because of its super-meta acronym), but experts insist that it's not to be taken lightly. "People with SAD can get severely depressed," says explains Norman E. Rosenthal, M.D., a clinical professor of psychiatry at Georgetown University School of Medicine and author of the authoritative book, Winter Blues.

RELATED: Brad Pitt Is Back and Sexier Than Ever

"Sufferers may lose relationships because they're not engaged enough with family and friends or get bad performance reviews at work because they're unable to execute their work."

RELATED: 14 Healthiest Snack Foods to Buy

During a recent study, researchers found that people with low vitamin D levels were at greater risk for developing SAD. The condition is more common in women than men, and is more prevalent in northern latitudes, due to limited winter daylight hours. However, Rosenthal explains, SAD can affect people living in, say, a shady part of Hawaii. And it can even flare up in some people during the summer.

Although there is no known cure for SAD, there are three key treatments that Rosenthal and other experts suggest: "Decrease stress and increase exercise," he says.

RELATED: The 7-Minute Total Body Workout

Phototherapy (a fancy word for, you guessed it, light therapy) has also been proven to be effective. "Go outdoors and take a walk on a bright winter day; the mornings are the best time for this," he adds.

RELATED: Shirtless Nick Jonas Explains How He Got His New Body

If you simply cannot make the time or there's a polar vortex crushing your part of the country, consider an Easy-Bake Oven approach: Sit under a light bulb. There's a marketplace full of special fluorescent or LED-based light boxes (costing around $150 and up) that are about 100 times brighter than usual indoor lights. "The ones that have been shown to work the best will have a beam that lights up one square-foot," Rosenthal says. These bulb-centric therapy sessions usually last 30 to 60 minutes. And no one is too busy to sit under a light—doing whatever you want to do—for a half hour.

Click for more from Details.