As the winter days get colder, some look forward to making snow angels and curling up with a mug of hot chocolate. But for millions of people, winter brings debilitating depression and lethargy. Seasonal affective disorder (SAD) is thought to affect 6 percent of the U.S. population, with millions more having milder forms of seasonal malaise. Here’s what you need to know about this condition.

1. Seasonal affective disorder is a relatively recent diagnosis.

Doctors have commented on the seasonality of depression in their patients for hundreds of years. The 19th-century psychiatrist Jean-Étienne Esquirol described a Belgian man whose life was generally good, but “at the beginning of autumn [he] became sad, gloomy, and susceptible,” and this pattern had continued for years. Esquirol prescribed a trip to the south of France and then into Italy as winter progressed. In May, the patient returned to Paris “in the enjoyment of excellent health.”

The modern understanding of SAD, however, didn’t emerge until the 1980s. A 1981 article in the Washington Post described a patient who was “almost dysfunctional in the winter, with both her mood and her energy levels at low levels.” It added that Norman Rosenthal, a researcher at the National Institute of Mental Health, “would like to hear from anyone with distinctly seasonal mood disorders. Applicants will be sent questionnaires, from which participants will be selected” for an experimental treatment program.

Decades later, Rosenthal told the Washington Post, “I thought I was dealing with a very rare syndrome. […] We got 3000 responses from all over the country.” In 1984 Rosenthal and colleagues identified SAD in the journal Archives of General Psychiatry, and in 1987 it was added to the American Psychiatric Association’s manual DSM-III-R.

2. Seasonal affective disorder doesn’t just happen in winter.

In the APA's current DSM (DSM-5), one benchmark for diagnosing depressive disorders “with seasonal pattern” is “a regular temporal relationship between the onset of major depressive episodes in major depressive disorder and a particular time of the year (e.g., in the fall or winter).” It also indicates that there must be no seasonally related stressors (such as consistent unemployment in winter), that full remission occurs at “a characteristic time of the year,” and that the pattern has repeated for two years without non-seasonal episodes.

Nothing in that definition requires winter, however. An estimated 10 percent of people with SAD experience the opposite of the conventional diagnosis—their depression appears in spring and summer. And in places like the Philippines, studies have found more people feel their worst in summer rather than in winter.

According to the National Institute of Mental Health, summer-onset and winter-onset SAD can even have different symptoms. Winter symptoms can include oversleeping, weight gain, carbohydrate cravings, and low energy, while summer symptoms might be poor appetite, insomnia, agitation, anxiety, and even violent behavior.

3. Seasonal affective disorder Isn't the ‘winter blues.’

SAD is not the same as feeling a little down as it gets gloomy outside. A SAD diagnosis meets all the criteria for major depression and should be treated as seriously—the only difference is that SAD has a seasonal pattern. Psychiatrists do recognize ‘winter blues,’ or sub-syndromal SAD (S-SAD), for “individuals who do not meet diagnostic criteria for depression during the fall/winter months, but who experience mild to moderate symptoms during fall or winter,” SAD expert Kelly Rohan told the APA. This form may affect an additional 15 percent of the US population. (This number is highly dependent on where the S-SAD patients live, however.)

4. Your chance of experiencing seasonal affective disorder depends on your latitude (to a point).

It might seem obvious that as you get farther north—to regions with colder, darker, and longer winters—SAD would be more prevalent. There is some evidence for this: An estimated 1 percent of Floridians experience SAD compared to 9 percent of Alaskans. But one study in Tromsø, in northern Norway, found “no significant differences in the reporting of current mental distress depending on season” (although they did find people had more sleeping problems in winter). Icelanders also have remarkably low instances of SAD. Even more surprisingly, people of Icelandic descent living in Canada have a lower prevalence of SAD than non-Icelandic Canadians in the same area [PDF].

5. Seasonal affective disorder doesn't affect everyone the same way.

SAD is reported to affect four times as many women as men, and a recent pilot study indicated vegetarianism may also be associated with SAD. The researchers found that:

“The percentage of SAD patients among Finnish vegetarians was four times higher than in the normal population. The percentage of vegetarians among the SAD patients in a Dutch outpatient clinic was three times higher than in the normal population. In the Dutch population, the seasonal loss of energy, in particular, is related to vegetarianism.”

Some factors may confound the data (for example, it’s possible vegetarians are more likely to forgo antidepressants, so there are more of them in outpatient facilities), but the researchers say the findings suggest a link. In fact, one theory for Iceland’s low SAD rate suggests that Icelanders’ fish-heavy diet may have a protective effect (and Icelanders living in Canada might be sticking with their traditional foods).

6. We don’t know what causes seasonal affective disorder.

While scientists haven’t figured out what factors cause SAD, the most popular theory is the phase shift hypothesis: That, due to later sunrises and earlier sunsets, the body’s circadian rhythms sometimes get out of whack with its sleep/wake cycles, like a several-months-long jet lag. It’s also possible that people with winter SAD can’t regulate serotonin, or they overproduce melatonin, and the imbalance alters circadian rhythms.

7. Luckily, seasonal affective disorder is treatable.

For years, the gold standard of SAD treatment has been light therapy. The process involves sitting near a light box for around 30 minutes after you wake up. Your eyes are open, but not looking directly at the light, meaning the therapy can be done while watching TV, reading a newspaper, or having breakfast.

But researchers warn again self-treating with light therapy—it can negatively affect people with bipolar disorder or eye problems. And, light boxes must be made specifically for treating SAD. Many commercially available light boxes release predominantly UV light, and SAD boxes should release as little UV as possible.

More recently, research has been looking into Cognitive Behavioral Therapy (CBT)—a technique that alters negative thoughts to manipulate emotions and behaviors, like changing your thinking from “I hate winter” to “I prefer summer.” With CBT, some researchers have seen fewer recurrences, less-severe symptoms, and higher remissions compared to light therapy users. Antidepressants are also prescribed for SAD.

8. Seasonal affective disorder may have once been an evolutionary advantage.

In the 1981 Washington Post article, the SAD sufferer commented that she “should have been a bear” because “bears are allowed to hibernate, and people aren’t.” As the years went on, some proposed that the symptoms of winter SAD—sleeping more, being less active, and eating fattening foods—could be a vestigial hibernation instinct. Many dismissed that explanation, but in the early 2000s things began to change. A Russian study found that women without depression and with non-seasonal depression consumed around the same amount of oxygen, while women with winter depression consumed less [PDF]. Doctors began to think SAD exists today because it once offered some evolutionary benefit for humans surviving winter.

One proposed benefit is reproduction. People with winter SAD are lethargic in winter but generally active in spring and summer, which increases the probability of procreation in those seasons. If a child is conceived between May and September, that means a high probability of being born between February and June, which some researchers propose would increase chances of survival before winter kicks in. Meanwhile, Robert Levitan at the University of Toronto “consider[s] SAD to be an evolutionary disorder, an energy-conserving process that is no longer helpful in modern society. While in modern times it’s not good for us to slow down too much in the winter, or to gain lots of weight, this probably helped our ancestors survive in the ice age.”