The days are getting shorter, darker and colder, at least in the Northern Hemisphere. And while there's a lot to look forward to in winter, it's not necessarily the most wonderful time of the year for everyone.

Many people have experienced the "winter blues" in some form or another. But in some cases, those changes in mood, sleep and appetite can be signs of something more serious: a form of depression known as seasonal affective disorder, or SAD.

"It's normal for people to feel a little worse in the winter," says Dr. Paul Desan, director of the Winter Depression Research Clinic at the Yale School of Medicine. "But for some people, these changes are really severe, and they add up to what is equivalent to a clinical episode of major depression."

About 5% of adults in the U.S. experience SAD, and it typically lasts about 40% of the year, the American Psychiatric Association (APA) says. Symptoms are most common in the fall and winter months, and include feelings of sadness, fatigue, cravings of carbs and starch and associated weight gain.

Desan tells Morning Edition that another roughly 10% of people suffer from subsyndromal SAD, in which they experience some symptoms when the seasons change but do not meet the criteria for clinical depression.

Doctors believe SAD is linked to the reduced sunlight exposure and circadian rhythm disruption that are hallmarks of the winter months — that's why you won't find much of it in Florida, Desan says.

One thing he doesn't see is a link between SAD and daylight saving time, contrary to popular belief (though the time change, particularly in the spring, can have its own negative health effects).

"For the most part, people adjust to the new clock, and we don't think Seasonal Affective Disorder would be eliminated if we eliminated daylight saving time," he adds. "Whether some people might be a little more sensitive to that jump in time, sure, that's possible."

Desan says even though we're somewhat independent of the outside light-dark cycle — thanks, in large part, to electricity — our bodies can still tell the difference between winter and summer.

"We act like we control our mood and our energy ourselves," he says. "But actually, environmental conditions affect us."

And that doesn't mean there aren't effective — and natural — treatments. Just as a lack of light can darken our moods, the right dose of brightness can make a big difference.

Who can get SAD?

Anyone can experience the symptoms of SAD, though some people might be more predisposed than others.

SAD typically starts when a person is between the ages of 18 and 30, according to the APA. Desan says people of all races can get it, though Scandinavian groups may be less prone.

Researchers believe the rates of SAD are about three times as high in women as in men, Desan says. He adds that women who tend to have more premenstrual mood changes are more likely to develop SAD, and vice versa.

Location — and especially latitude — play an important role, too. SAD is more common in people living far from the equator, Desan says, which is why its rates are higher in the northern part of the U.S. and Canada and lower in the south.

Desan says there are a whole bunch of potential factors, and the "excitement of psychiatry is that all of these things add up."

"If everything is going wonderfully in your life, maybe you're going to be a little less susceptible to the darkness of winter," he explains. "On the other hand, if you're under different kinds of stresses, I think you are more susceptible."

Desan says some people might minimize or dismiss their symptoms, especially if they're hearing from those around them that it's normal to feel worse in the winter. That's unfortunate, he adds, because both SAD and subsyndromal SAD can be very effectively treated in the majority of patients.

How is it treated?

If you think you may have SAD — especially if you're experiencing significant depression or having thoughts of suicide — you should make an appointment with a mental health professional, Desan says. They can assess your symptoms, make a diagnosis and supervise a course of treatment if necessary.

"If your depression is that serious, don't mess around with treating yourself," he adds. "Get someone who's qualified to treat you."

For some people, he says, antidepressants might be the right answer. But many patients prefer to try — and have seen success — with a more natural approach.

He says the standard starting point is a bright light treatment, in which the patient is exposed to bright light first thing in the morning, ideally before 8 a.m. or even earlier. The light box should be 10,000 lux (a measurement of the intensity of light), which Desan compares to being "outside in July in the middle of the day."

Patients should sit in front of the light — about a foot away from it or more, depending on its exact size and brightness — for about 30 minutes (even if they eventually drop down to 15), at the same time each morning, seven days a week. He says the improvement can be dramatic, even if it takes several weeks to reach its full effect.

"For many of our patients, this is plain and simple, a miracle," Desan says. "This is a life-changing therapy for many people with Seasonal Affective Disorder or subsyndromal Seasonal Affective Disorder."

These devices go for as little as $80, Devan says, though are not likely to be covered by insurance. His clinic keeps a list of some of the models that meet its guidelines.

He recommends investing in a device that's big and bright enough, because it will play an important role in a patient's morning routine.

"Maybe you're going to turn the device on at 7:00 on a table or countertop and you're going to eat breakfast, read the paper, listen to NPR or whatever you do in the morning," he adds. "So you want a device that's big enough that you could sit at a reasonable distance and move around a little bit and still get that exposure."

There are other lifestyle changes that might help. The American Medical Association recommends sleep hygiene, stress management, physical activity and spending time outside. Doctors may also recommend vitamin D supplementation for certain patients, depending on their levels.

Check out NPR's Life Kit for more tips on how to recognize and cope with the symptoms of SAD.

If you or someone you know is in an emotional crisis, reach out to the National Suicide & Crisis Lifeline by dialing or texting 988.

The broadcast interview was produced by Iman Maani and Ana Perez, and edited by Jan Johnson.