If you notice drastic shifts in your mood during certain times of the year, you could have seasonal affective disorder. Here are answers to your top questions about the condition.

The fall and winter months often signal the season of celebration and holidays. But for many, the darker, colder days herald another type of period: one of fatigue, lethargy and depression.

Seasonal affective disorder, or SAD — a type of depression that occurs with the changing of the seasons — can affect up to 10 percent of people in the United States, depending on where they live. Here are some of the most frequently asked questions about seasonal affective disorder, with answers from experts.

“One big, common misconception about SAD is that it’s just the ‘winter blues,’” said Kelly Rohan, a professor of psychological science and a clinical psychologist at the University of Vermont. When the days get grayer and colder and the holidays are around the corner, it’s not unusual to experience temporary feelings of malaise, tiredness or stress. But seasonal affective disorder is much more serious — a form of clinical depression, Dr. Rohan said — with symptoms that can last longer, and that come and go with the seasons.

According to the National Institute of Mental Health, you may have seasonal affective disorder if you experience certain symptoms that start and stop during specific months, and that occur for at least two consecutive years. These can include near-constant and daily feelings of depression, loss of interest in activities you used to enjoy, changes to your appetite or weight, sluggishness and low energy, trouble falling asleep or oversleeping during the day, or even thoughts of death or suicide.

Most people who have seasonal affective disorder have winter-pattern SAD (or winter depression), where symptoms begin in the late fall or early winter and go away during the spring and summer. But SAD can also occur during the spring and summer months, called summer-pattern SAD (or summer depression).

SAD is much more common in women than it is in men, and in those living in the northern regions of the United States — like Alaska or New England — than in those living in Southern regions, like Florida. It can also sometimes run in families.

“People may not appreciate how severely someone who has SAD is affected,” said Dr. Paul Desan, a psychiatrist at the Yale School of Medicine. For some people, he added, “their life just shuts down for half the year.”

For those with winter SAD, shorter, darker days are what trigger depressive symptoms, Dr. Rohan said. But experts don’t know exactly why this happens.

A leading theory has to do with a shift in the biological clock. Normally, the body produces melatonin at night, which helps promote sleep. When the levels of melatonin taper off as sunrise approaches, that helps people wake up. But if you have winter SAD, melatonin peaks later and lingers for longer into the morning, making it harder to wake up and leaving you fatigued and groggy. Because you don’t reach peak wakefulness until later in the day, it’s harder to fall asleep once evening comes — perpetuating a cycle of insomnia, inadequate sleep and fatigue, and exacerbating depressive symptoms.

Most people who are vulnerable to SAD are most likely always susceptible to feeling down, said Kathryn Roecklein, an associate professor of psychology at the University of Pittsburgh. But in the winter, light levels fall below a threshold and suddenly things are off balance.

There’s also some evidence that the negative anticipation of shorter days — thinking, for instance, “I just don’t function well in the winter” or “I’m going to start feeling poorly soon,” Dr. Roecklein said — can combine with biological effects to create a self-fulfilling prophecy.

While this can vary from person to person, SAD symptoms can linger for quite a while — up to five months for some, said Dr. Lily Yan, a professor of psychology and neuroscience at Michigan State University.

Most people who have winter SAD will begin noticing symptoms between October and November, Dr. Rohan said. But if you live in parts of the country where people are more susceptible, you may notice them earlier in the fall. Stressful life events, like work instability or grieving a loved one, might also kick off depressive episodes earlier than usual.

The most severe symptoms tend to occur in January and February, Dr. Yan said. Some experts think this happens because of the accumulated effects of SAD over time, she said. The longer you have symptoms, the worse they get. But people tend to start feeling better once spring and early summer roll around.

Symptoms of summer-pattern SAD, which is much less common, tend to start in the spring and end around the beginning of fall.

The good news, experts say, is that there are several evidence-backed ways to find relief.

“Bright light treatment first thing in the morning dramatically improves the vast majority of people with seasonal affective disorder,” Dr. Desan said. This involves sitting in front of a light therapy box, which mimics outdoor light, for about 30 minutes every day right when you wake up, preferably early in the morning. This should stimulate your body to produce the right hormones to increase your wakefulness and alertness to get you through the day, Dr. Desan said.

Just make sure to use it properly. If you don’t use it early enough, it may not work; and if you use it too late in the day, it could exacerbate insomnia, Dr. Rohan said. Most experts recommend using light therapy lamps before 8 a.m.

Plenty of lamps out there won’t do you any good, Dr. Desan said. When searching for a light therapy lamp, you want one that advertises 10,000 lux brightness — the equivalent of a bright summer morning. A good light therapy lamp should also be “full spectrum,” he said, meaning it emits light that closely mimics natural morning sunlight. Unfortunately, it’s challenging to know exactly what you’re getting, since light therapy lamps aren’t regulated. To help guide buyers, Dr. Desan’s lab at Yale has clinically vetted a number of lamps, and listed its recommendations on its website.

If you think you have seasonal affective disorder, consult with a therapist or psychiatrist before deciding on a treatment plan. They can offer guidance on the best plan for you — and if it involves bright light therapy, they can teach you how to do it properly, Dr. Rohan added.

C.B.T., a form of talk therapy that aims to shift faulty or unhelpful ways of thinking, can also be beneficial, Dr. Rohan said. If you say things like, “I hate winter” during a session, she said, the therapist will try to reposition those negative thoughts. “Even if we could take it down a notch to, ‘I prefer summer to winter,’” she said, that can result in a big shift in symptoms and mood.

Because cognitive behavioral therapy for SAD involves retraining your negative associations with darker months, it is the only treatment that could have long term benefits after doing it just once, Dr. Roecklein said. “It works in the winter when that person comes in for a treatment,” she said, “but then it also protects them from depression the next winter and the winter after that.”

Seasonal depression can also be treated with antidepressant medications, like the selective serotonin reuptake inhibitors fluoxetine (Prozac), citalopram (Celexa), sertraline (Zoloft), paroxetine (Paxil) and escitalopram (Lexapro).

Bupropion (Wellbutrin), another type of antidepressant medication, is the only drug approved by the Food and Drug Administration to prevent recurrences of seasonal major depressive episodes. If prescribed this medication for winter SAD, you would take it once a day from fall to early spring.

If you’re tempted to try certain supplements like folic acid, B12 or vitamin D, don’t waste your money, Dr. Roecklein said. Unless you have a true deficiency, she said, there is “zero evidence that taking supplements” will work for treating or preventing seasonal affective disorder.

Simply going outside can also help boost your mood, Dr. Yan said. Even if it looks a little gray, the quality of light on a winter morning will be better than what you can get in your home. And an excursion will most likely boost your mental health too, she added.

Replacing your favorite summertime activities with similar wintertime activities can “bring a sense of enjoyment into life,” Dr. Rohan said. If your favorite summer activity is lounging by a lake, she said, you can still do a version of that — maybe strap on some snowshoes and walk around, or try skating or sledding. “It may take some creativity and problem-solving,” she said, but it will help you find a way to positively engage with the season.

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