Seasonal affective disorder: Can light therapy treat SAD?

Each winter, I get a brief respite from New York City’s sad, gray days and 4:30 p.m. sunsets by visiting my parents in Florida for a few weeks. Suburban Florida is not a very interesting or exciting place, but there’s something about seeing palm trees and blue skies in December that feels energizing.

On my most recent trip. I commiserated with a friend who was visiting from Montreal and, like me, feels inexplicably sluggish in the colder months. He suggested I buy a “happy lamp,” a bright light often used to treat seasonal affective disorder, to help me feel more rested. “I use it every morning, and it’s made winter bearable,” he told me.

I was skeptical that shining a bright light in my face would make me feel better, even though people do it all the time. And even if it did work, I assumed it’d address the symptoms of my winter lethargy without fixing whatever the underlying problem was.

But light therapy, it turns out, is actually very effective. Katie Sharkey, an associate professor of medicine, psychiatry, and human behavior at Brown University’s Alpert Medical School, told me she often prescribes light therapy to patients diagnosed with SAD. “I think it’s very effective,” Sharkey, who specializes in psychiatry and sleep medicine, said, “but I would recommend that people who have depressive symptoms get seen by someone who can collaborate with them on this treatment, just like you would with a pill or therapy.”

In other words, light therapy works — but you probably shouldn’t buy a SAD lamp just because someone at a party told you to.

What is seasonal affective disorder?

To understand how light therapy works, first you have to understand what causes seasonal affective disorder.

As its name suggests, seasonal affective disorder is a “depressive disorder or mood disorder that occurs most often in the winter,” Sharkey told me, though psychiatrists also have records of patients suffering from summer depression. “It’s part of a cluster of syndromes where mood is affected by day lengths.”

Symptoms are similar to those for year-round depression and include persistent low mood, a loss of interest in daily activities, irritability, finding it hard to get up in the morning, feeling lethargic throughout the day, and changes in appetite, which can mean eating too much or too little but often manifests as cravings for “certain heavier-calorie foods.” Somewhere between 1.4 and 9.7 percent of Americans have SAD, with higher rates the further north you go.

As Joseph Stromberg wrote for Vox, scientists used to doubt the existence of SAD, which was first identified in the 1980s, and initially believed it was caused by insufficient exposure to sunlight in the winter. The idea, which has since been debunked, was that shorter days led to excess production of melatonin, a hormone that regulates sleep-wake cycles, making people feel groggy and irritable during the dark winter months. This was called the photoperiod hypothesis.

Another theory, the phase-shift hypothesis, posits that since the sun rises later in the winter, humans’ circadian rhythms stop being aligned with their sleep-wake cycles. Put more simply, since people tend to wake at the same time every day, even in the months when the sun rises later, change in sunrise times can throw their circadian rhythms out of whack.

But a pair of new studies, both published last September, suggest there’s another explanation for seasonal depression: a brain circuit that connects light-sensing cells in your eyes to regions in your brain that affect your mood. One study found this type of circuit in mice; a few weeks later, a different study suggested a similar pathway exists in humans. “It’s very likely that things like seasonal affective disorder involve this pathway,” Jerome Sanes, a professor of neuroscience at Brown University, told NPR in December.

Samer Hattar, one of the authors of the mouse study and the chief of the section on light and circadian rhythms at the National Institute of Mental Health, made a discovery in 2002 that helps explain these recent developments. In 2002, Samer and Brown neuroscience professor David Person discovered a new type of cell in the eye. Combined with other work done on SAD, this suggests that seasonal depression is caused by a lack of light.

“Before the cell paper was published [in 2002], a lot of people — including me, to be honest — just made the assumption that the depressive effects of light are [caused by] disruption to the circadian clock,” Hattar told me. “They assumed that the sensitivity of this system was similar to the sensitivity of the circadian system. But now that we’ve found this completely new circuit, we don’t know if the sensitivity of the system is the same as the circadian system. We don’t know how much light — for humans — you need to shine to activate this mood-enhancing pathway.”

That doesn’t mean light therapy isn’t effective. “People have known for many years that light therapy works,” Hattar told me. Even if scientists are still trying to figure out what causes SAD, there’s nearly a consensus that light therapy is an effective treatment that could address SAD directly, not just its symptoms.

SAD lamps work — if you use them the right way

Light therapy, or phototherapy, was prevalent long before seasonal affective disorder was considered an actual affliction. According to the National Library of Medicine’s blog Circulating Now, the practice was used in antiquity to help balance a person’s humors — this was back when doctors thought that all medical problems were caused by imbalances in four bodily fluids (black bile, yellow bile, phlegm, and blood), a bizarre form of pseudoscience that endured for more than 2,000 years — and was later used to treat a range of conditions including psoriasis, lupus, and sleep disorders.

In 1903, physician Niels Ryberg Finsen was awarded a Nobel Prize for his work on the therapeutic and physiological effects of light treatment. A few decades later, sun lamps became commonplace in both Europe and the US — both for therapeutic and cosmetic purposes.

In Sweden, where the sun sets around 3 pm in January, light therapy clinics have been a popular way of treating SAD since the 1980s. Some Swedish light therapy clinics, writer Linda Geddes wrote for the Atlantic in 2017, even dress patients in all-white outfits and have them sit in whitewashed rooms full of bright light to treat the winter blues. According to Geddes, light therapy clinics are no longer as prevalent was they were in the ’80s, and only a few remain today.

But people who find themselves suffering from winter-induced depression have other treatment options that don’t require sitting in blindingly white rooms. Light boxes, or light therapy lamps, have become increasingly popular in recent years, especially in the US, and even among people who haven’t been officially diagnosed with SAD.

Google searches for “lamp for seasonal affective disorder” have skyrocketed since 2016, according to Google Trends — perhaps not surprisingly, people tend to look into buying these products in November and December, and people in northern states with rough winters, like Minnesota and Michigan, are the ones driving the most search traffic. The Cut, the Wirecutter, and CNN have compiled lists of the best SAD lamps, which range from sleek light boxes that fit on your desk to floor lamps.

Sharkey, the Brown University psychiatrist, said light therapy can be a very effective treatment for patients who have been diagnosed with SAD, adding that Hattar’s and Sanes’s respective studies were a huge breakthrough in understanding how SAD works. The mouse study, she said, was “probably the most exciting paper I’ve seen in the last year.”

“It’s huge, because it provides a neural substrate for what we’ve observed clinically” — that natural light can have huge effects on a person’s mood, especially in the winter months. “The artificial light we’re under is not giving the same signal to our brains as the light we get outdoors,” Sharkey told me. “Even though it may seem slightly less bright than outdoor light, to our brains it’s actually much less bright. I’m sitting in my office now: I can see perfectly, I can read. But if I went outside, even though it’s overcast here, it’s still orders of magnitude brighter outside to the brain,” she said.

Light therapy can more or less trick your brain into thinking you’re getting hit by natural light, as long as you’re using the right device. “There are features that you should definitely look for,” she said. “I usually tell people that they want a light box that’s got UV filtering, that can produce with one and five thousand watts. I tell patients that a broad-spectrum light box, rather than one that’s a particular color or hue, is usually better because people tend to find those more pleasant. Those are some of the features I look at. If you [find one that meets] those three criteria, then it can be based on aesthetic preference and price range. You can spend 300 bucks or 50 bucks, and you should spend the number of bucks you can afford.”

But, Sharkey warned, light boxes have to be used correctly — use it too early in the day and you’ll find yourself getting tired before your usual bedtime; use it too late and you’ll have trouble getting to bed — and they can lead to other side effects, like headaches, interference with certain medications, and rashes. And, she added, this decision should be made in consultation with a professional.

“I definitely recommend that people who feel depressed enough to seek treatment seek treatment,” she said. “That’s a rule to live by: Take care of your mental health.”