Between summer solstice in June and winter’s near Christmas, our daylight dwindles by half. In November, we average about nine and a half hours of daylight per day. It’s not uncommon to feel the effects of the cold and darkness and waning of light this time of year.

An article from the New York Times covers seasonal affective disorder, or SAD—a type of depression that occurs with the changing of the seasons— that 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.

What is season affective disorder?

“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 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.

What causes season affective disorder?

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, according to 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.

How can season affective disorder be managed best?

Bright light therapy

“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.

Cognitive behavioral therapy (C.B.T.)

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.”

Antidepressant medications

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.