The science behind seasonal depression

When he moved from South Africa to New York City, Norman Rosenthal noticed he felt more depressed during the cold, short days of the city’s winters than he had in his home country.

“It was an illness hiding in plain sight because people said ‘well that’s how everyone feels in winter.’ They didn’t see it as treatable,” says Rosenthal, a psychiatrist at Georgetown Medical School. 

In 1984, he published the first paper to scientifically name the winter blues: Seasonal affective disorder (SAD), also called seasonal depression, was a type of depression brought on by the dark days of winter. 

Subsequent studies have found that this form of depression varies by geography. As much as three percent of the general population is thought to experience SAD, but one study Rosenthal published in 1990 found that the condition became more prevalent in the U.S. in northern latitudes, with as many as 10 percent of New Hampshire residents reporting the condition.

And, surprisingly, about 10 percent of patients suffering from SAD have symptoms in the summertime instead. 

Whether in winter or summer, mental health experts say there are solutions to treat SAD. 

A bad mood versus a SAD mood

It’s normal for moods to fluctuate with seasons and even for people to feel a little more down in the winter, experts say, but those suffering from SAD experience the symptoms of clinical depression. 

“They’re exactly the same,” says Kelly Rohan, a psychologist at the University of Vermont who specializes in the disorder. 

“We would look for things like a persistently sad mood. Losing interest in things. Sleep changes. Significant eating or appetite change. Losing energy. Fatigue. Difficulty concentrating,” she says. 

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At Yale’s Winter Depression Research Clinic, the most commonly reported symptoms of winter depression are hypersomnia—the desire to sleep more than usual—and an increased appetite, says Paul Desan, a psychiatrist and the clinic’s director. 

“It’s like human beings are trying to hibernate,” says Desan.

Most people begin experiencing symptoms in young adulthood, but SAD can begin at any stage of life. The condition also varies by sex. 

“About three times as many women as men get SAD for reasons we don’t understand,” says Desan. 

While SAD is typically diagnosed when patients meet the criteria for clinical depression, some patients fall just short of this official diagnosis. They’re instead diagnosed with “subsyndromal SAD,” a milder but still burdensome condition. 

Why does SAD happen? 

“It’s fairly accepted that the trigger is a short photoperiod—the number of hours from dawn to dusk,” says Rohan. “It’s the strongest predictor of when the symptoms begin.” 

But why, exactly, does less daylight seem to make some people clinically depressed? 

“That's the million-dollar question,” says Rohan. 

One leading theory is that the change in daylight disrupts our bodies’ release of melatonin, a hormone the brain releases every night to promote sleep. The process is part of our circadian rhythm, our innate biological clock dictating when we sleep and wake.

(Are you a lark or an owl? Your body clock holds the answer.)

Our clocks are cued up by morning light, but as the supply of daylight dwindles in winter, melatonin may be released later and wear off later, the theory suggests. 

That means, “that alarm clock might be going off in the morning, but the body is still in a state of biological night,” says Rohan. 

Those diagnosed with SAD may be particularly sensitive to schedules that disrupt their circadian rhythm by beginning and ending at odd hours, like shift work, she adds.

Even given that theory, she says it’s still unclear why disrupted circadian rhythm can lead to depression, instead of just feeling fatigued. 

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What about summertime depression? 

Rohan says a minority of people who experience SAD in the winter can become manic in the summer when the number of stimulating daylight hours increases. 

But still others experience a phenomenon called summer SAD, or when seasonal depression symptoms occur only in the summer—perhaps triggered by the heat and humidity. 

Instead of feeling lethargic, summer SAD patients more often feel irritable and agitated. 

(Learn more about why seasonal depression can happen in the summer.)

Scientists are studying whether temperatures rising as a result of climate change may harm mental health. A 2018 study of mood expressed on social media found depressive language increased with temperature. 

Is SAD a real condition? 

SAD is not without its skeptics. One study published in 2008 looked at rates of depression among populations in northern Norway, where there’s no sunlight at all for two months of every winter, and found no seasonal increase. 

Another study published in 2016 looked at a U.S. survey of just over 30,000 people. When asked to answer questions related to depression, there was no discernible spike related to season or latitude. 

Steven Lobello, a psychologist at Auburn University in Montgomery, Alabama, isn’t convinced SAD qualifies as a diagnosable mental disorder. He thinks previous studies done to measure rates of SAD have been too suggestive. Rather than calling people during the winter and asking them if they feel depressed, studies have asked people if they have ever felt depressed during winter—which many people believe they do. 

“If you ask questions of people in a way that allows them to know the nature of what you’re getting at, then I don't think you’re doing anything more than measuring that belief,” Lobello says. 

However, Rohan says these studies may not detect SAD because it’s relatively rare, representing about 10 percent of those diagnosed with clinical depression. 

“It’s like looking for a needle in a haystack and not finding needles and determining they don’t exist,” she says. 

Using light as therapy

For those who think they may be experiencing SAD, experts say a professional diagnosis is a crucial first step toward treatment. 

“People should really avoid self-diagnosis,” says Rohan. “Depression is a serious mental health problem, so it’s best to leave it to trained professionals.” 

Sitting in front of a bright box of light is one of the most common treatments for SAD. Experts say the key is to look for light boxes that provide light equalling 10,000 lux, a measure of brightness. 

“10,000 lux is like being outside on a summer day in July. It’s a lot of light,” says Desan. 

Be wary of the box’s size, he notes. A light box that’s too small can fail to deliver the adequate amount of light treatment. 

(His clinic’s website lists vetted boxes.) 

Experts recommend sitting in front of light boxes for about 30 minutes and usually first thing in the morning, to help signal to the body that it’s time to wake up. This treatment can improve mood the day it’s used. The only drawback to light boxes, notes Rohan, is that they only work on the day that you use them, so they need to be used daily. 

Talk therapy and lifestyle changes

One treatment that may have longer lasting benefits is Cognitive Behavioral Therapy (CBT), a form of talk therapy Rohan endorses for treating SAD. 

“Negative thinking tends to breed negative emotions, and we want to change those into slightly less negative, more neutral thoughts,” she says of the CBT approach. For example, “I hate winter,” might be reframed as, “winter isn’t my favorite season, but I still find things to enjoy.”

Finding wintertime hobbies may also help. 

“People with SAD often have hobbies and interests that are summer specific—growing gardens, beach going,” she says. Instead of hibernating under a blanket, she suggests those people find indoor hobbies to enjoy—knitting, joining a book club, or going to the gym, she suggests. 

Lifestyle changes can also be useful mental health tools, says Rosenthal. Exercising, learning ways to manage stress, or planning a sunny vacation during the winter can all help to boost your mood, he says. 

Whether using light or talk therapy, Rosenthal stresses that there’s no reason to not seek mental health treatment, even if symptoms are only present for a few months out of the year: “There are many, many strategies people can use that will help them thrive year-round.”