For about 1 in 20 people in the northern half of the United States, cooling temperatures and shorter, darker days may signal the onset of seasonal affective disorder, or SAD, a type of depression that typically arrives in the fall or winter, then goes away in the spring.

Unlike mild cases of the “winter blues,” SAD symptoms make it difficult to function. It tends to start with so-called “vegetative symptoms”: an increased appetite and a craving for carbohydrates like french fries or ice cream, the urge to sleep longer hours, difficulty getting up in the morning and feeling wiped out at work.

Then, in three to four weeks, “the mood plummets,” said Michael Terman, a professor of clinical psychology at Columbia University and an expert in seasonal affective disorder. Patients with SAD develop major depression, which includes persistent feelings of sadness, withdrawal from friends and family and a loss of interest in activities that were once enjoyable.

Researchers don’t yet know why some people develop SAD and others do not, but the disorder is believed to run in families and is more common among women. SAD develops in the fall and winter because shorter daylight hours and less sunlight shift the body’s internal clock, and certain mood-regulating hormones, like serotonin, oscillate with the seasons.

The good news is that because SAD is tied to the changing seasons, “you can predict its onset and ward it off,” Dr. Terman said.

If you have already started experiencing vegetative symptoms — for example you are sleeping longer and having more difficulty waking up — or if you already know you are susceptible to seasonal affective disorder, experts said it’s best to start implementing preventive measures before major depression sets in.

Bright light therapy, which typically involves sitting near a light box for about half an hour each morning, is considered the most effective treatment for SAD, several specialists in the field said.

Two meta-analyses of randomized, controlled trials demonstrated that bright light therapy was superior to a control when treating SAD. And one study found that over the course of four weeks, 61 percent of the 33 patients who received bright light therapy saw their symptoms disappear compared to 32 percent of the those who received a placebo.

Bright light therapy usually works more quickly than medication, said Dr. Terman, president of the Center for Environmental Therapeutics, a nonprofit that researches nonpharmaceutical interventions and teaches people how to use them. “It does, however, require consistent use upon waking.”

The Center for Environmental Therapeutics has a handy tool that can help you figure out the optimal time of day to do light box therapy based on your circadian rhythms. But for many, and especially those who are currently experiencing depression, it will be best to also seek the guidance of a health care professional.

Light boxes are not regulated by the Food and Drug Administration, so consumers need to do some research to make sure they are purchasing one that clinicians would consider effective.

“Most of what you find on the web is garbage,” said Dr. Paul H. Desan, an associate professor of psychiatry at the Yale School of Medicine who has studied seasonal affective disorder for decades.

An ideal box will have a large screen that shines light downward, akin to how the sun shines from the sky. The light should have an intensity of 10,000 lux, which is a measure of how much light is received by the eye. Lux declines the farther that you are from the light source, the experts said.

A 2019 study by Dr. Desan and his colleagues examined 24 light boxes and found that only seven of them met the investigators’ clinical criteria. Some advertised 10,000 lux, for example, but only produced this intensity “at unreasonably close distances, over a restricted field or with unacceptable glare or unevenness of illumination.”

Dr. Desan and his collaborators named six reputable models on their website, currently ranging in price from $129 to $369. (But keep in mind that this is not an exhaustive list and there may be other models that are just as effective that were not included in the study.) Other models can be found in this guide from Wirecutter, which is owned by The New York Times.

Bright light therapy is typically done during late fall and winter, but it can also be used at other times of the year. Dr. Desan recommended that anyone beginning treatment for SAD spend 30 minutes in front of their light box every day before 8 a.m. for at least three weeks.

“If it hasn’t helped at that point, it’s probably not going to help,” he said.

Some people may also benefit from medication, such as an antidepressant.

If you have eye-related concerns or a physical condition, like diabetes, that predisposes you to eye disease, then it’s best to check with your doctor before using bright light therapy.

Dawn simulation devices, which help you wake up by gradually increasing the amount of light in your bedroom, were shown to be even more effective than a light box in a 2001 study, however Dr. Desan cautioned that he knows of no device currently on the market that has been solidly supported by research and accepted by experts. Dr. Terman agreed.

Another type of device called a negative ion generator has shown some promise — high doses of negative ions have been associated with lower depression scores in some studies. But additional research is needed to further evaluate the efficacy of these devices in treating seasonal affective disorder.

Some people assume that taking vitamin D supplements can help cure seasonal affective disorder. But studies indicate that is not the case, so there is no need to take additional vitamin D unless a blood test shows you have a deficiency.

Finally, while bright light therapy is the first recommended treatment for SAD, getting outside regularly can offer other opportunities to soak in some light.

“You might want to retreat like the hibernating bear, but don’t,” said Dr. Norman E. Rosenthal, the psychiatrist whose research team identified and named the disorder in the 1980s. His book “Winter Blues” outlines strategies to beat seasonal affective disorder.

“Walking outside even 20 or 30 minutes each day could make a huge difference,” he added.

For ideas, look to Scandinavia, where various cultural practices offer cheerful ways to get through the winter.

Norwegians have a custom called koselig, which is about creating a cozy, happy environment with the people you love — and it includes spending a lot of time in nature. Participating in outdoor activities like skiing, snowshoeing or hiking will get you out of your house, into the sunlight and socializing with others.

You might also borrow some inspiration from the Danish custom of hygge and aim to make your home a more comfortable space to spend the chilly days ahead. The Swedish version is called mys. Think crackling fires, soft throw blankets, fuzzy socks, warm foods and good times with friends.

Candles are especially vital in Scandinavian culture. “Where Americans see a fire hazard, the Danes see an antidepressant,” wrote Penelope Green, a reporter with The New York Times, in a 2016 article about hygge. In fact, she noted, the Danish word for spoilsport is lyseslukker, “which literally means, ‘one who puts out the candles.’”

In essence, winter can become a time “for cultivating internal joys,” Dr. Rosenthal said. “Once you have your SAD under control, it becomes possible.”