It’s that time of year again. The clocks have gone back, it’s pitch black as you finish work, the cold, dark, dreary weather swoops in. And so do the memes about how depressed you’re going to be.

As the days grow darker earlier in northern latitudes, an affliction takes hold. Not just run-of-the-mill discontent over darker evenings, but the makings of an actual mental illness. Seasonal affective disorder, or SAD, is estimated to affect about 10 percent of people in northern latitudes. It’s often marked by low mood, a craving for carbohydrates, and fatigue that persists despite sleeping too much, lasting throughout an entire season. Women are estimated to be about three times more vulnerable than men. A whole industry has blossomed to treat it with light therapy, and it’s even made it into the courtroom.

But, despite affecting so many people, the very existence of SAD remains a point of contention.

The condition was first described in 1984 in the journal JAMA Psychiatry by Norman Rosenthal, a South African psychiatrist. The inspiration came from his own temperament: After moving from South Africa to New York in 1976, Rosenthal noticed that he had lower energy and productivity during wintertime. When the snow began to melt, his productivity levels rose once again.

Around the same time, during the second year of a psychiatric research fellowship, Rosenthal met Herb Kern, a scientist who had documented the seasonal patterns of his depression for years. Rosenthal and colleagues decided to try to treat Kern’s condition with light therapy—this entails using light boxes to stand in for sun rays, the idea being that they would be extending the length of his day with artificial light. It worked.

After a 1981 Washington Post article described their research, thousands of people got in touch, describing a similar winter-timed malaise. Rosenthal and his colleagues collected enough for a study of 29 bipolar patients in Maryland. They again tried treatment with light therapy—with success. (In an interview in 2020, Rosenthal said that the on-the-nose shorthand for the condition soon followed because they were looking for a “snappy acronym.”)

Three years later, in 1987, a seasonal pattern in depression was included in the Diagnostic and Statistical Manual of Mental Disorders, or DSM, often called psychiatry’s bible. But SAD is not listed as a stand-alone condition, rather as a type of recurrent major depression that emerges during a specific season every year. (There’s also a subcategory of SAD, the less severe version of seasonal affective disorder commonly known as the “winter blues.”) The most common subtype of SAD happens in the winter, although it can happen as other seasons usher in, including in the summer.

Researchers have suggested that SAD reared its head when humans began to move away from the equator and hours of daylight were curtailed. It’s unclear what exactly triggers the condition, but it’s thought to be a mixed bag. Less sunlight messes with your circadian rhythm, pushing it out of whack. So it goes, this imbalance can in turn affect your neurotransmitter levels. Or people could be struggling to make enough serotonin, which is responsible for managing their mood. SAD sufferers could be overproducing the hormone melatonin, making them sleepy.

But SAD’s existence hasn’t gone without questioning. “It has been much more embraced by the general public who identify with the symptoms than by the medical profession,” Rosenthal said in 2014.

In 2016, a controversial paper published in the journal Clinical Psychological Science stirred up a debate. Steven LoBello, a professor of psychology at Auburn University at Montgomery in Alabama, had needed a thesis idea for his graduate student, Megan Traffanstedt. The US Centers for Disease Control and Prevention regularly collects behavioral data by phoning members of the public, and LoBello had been working with the data sets, having done a massive collection of data using the PHQ-8 Depression Scale, a depression screening tool.

“I got to thinking: Well, you know, this would be really an unprecedented amount of data for documenting the seasonal effect on depression,” LoBello says. They set about analyzing the data. But when they measured depression levels against season, latitude, or sunlight exposure, they found no association. LoBello says they double-checked their methodology; they still couldn’t find why their results differed so heavily from others in the field.

In 2019 they completed another study, which again found no link. LoBello attributes the phenomenon of seasonal depression to being a “folk psychological construct.” ”It’s been continuously kind of stoked by media interests and that sort of thing,” he says. “I think, in fact, it would be devilishly hard to prove something like this.”

But LoBello’s work was met with pushback. Criticisms included that the PHQ-8 does not capture the full spectrum of depression, especially the slightly abnormal symptoms of SAD, like heightened carb cravings or oversleeping, and therefore might be missing cases. Kathryn Roecklein, an associate professor of psychology at the University of Pittsburgh, says the analysis didn’t find a significant link because the study “wasn’t designed to pull out a subgroup that does have seasonal variation,” she says. If there’s more people with depression that doesn’t have a seasonal variation in the group, it’ll cancel out those that do.

But LoBello’s research was not the only work to find no link. In 2019 a group of international researchers looked through the literature and again found the link to be lacking. “We did not find convincing evidence for seasonality effect in depressive symptoms at the population level,” they wrote.

The research they looked at did contain some peculiarities. In one twist, a country at a high latitude like Iceland should theoretically be full of SAD sufferers. But a 2000 study found no noticeable difference between the rate of anxiety and depression in winter versus summer. A 2020 study from researchers in the Netherlands looking at the Dutch population found that mood dips in winter were present only in people who already displayed high degrees of neuroticism, defined as being more prone to negative emotions.

One point of criticism is that SAD is diagnosed using a questionnaire called the Seasonal Profile Assessment Questionnaire, or SPAQ, which has been criticized for not being specific enough. The questionnaire works by retrospectively asking people whether they felt mood shifts through the seasons, meaning it may suffer from recall bias. “I don’t think that that instrument measures depression,” says LoBello. “It basically asks people what season of the year their mood changes.”

But just how prevalent SAD actually is doesn’t actually matter, Roecklein argues. “Every single disorder in psychology is based on self-report,” she says. “We’re not going to help anyone clinically by debating the timing of their episodes.”

What’s more, the three treatments that are effective for SAD—bright light therapy, cognitive behavioral therapy, and antidepressants—are also effective for nonseasonal depression. “To a large extent, separating the groups is less and less useful,” says Roecklein.

One advantage to knowing if someone suffers from a seasonal form of depression is being aware of what their depression trigger is. For some people, that could be a stressful life event. For others, it might be the change of the seasons.

SAD might not even be a forever thing. One theory behind the trigger of SAD is our reliance on artificial lighting on dark winter evenings. This extended exposure to light at nighttime may be wreaking havoc with our circadian rhythms. But humanity has been using artificial light for about a century. “A hundred years of artificial light has not allowed us enough time to adapt,” says Roecklein. Maybe wait a couple thousand years for evolution to catch up, and the phenomenon of seasonal depression could be a thing of the past, real or not.