Seasonal affective disorder can last for months. Here are the signs.

“It’s an annually recurrent depression. Rinse, repeat, do it again the next year,” said Kelly Rohan, a clinical psychologist and professor of psychological science at the University of Vermont. “That is a lot of time in depression.”

There is no simple answer as to what causes SAD. For some people, darker, dreary weather just has a negative psychological association, experts said. But there is also evidence that the reduction in sunlight during the fall and winter both directly and indirectly leads to changes in mood.

What are the signs of SAD?

One sign of SAD is a change in sleep habits, mainly excessive sleep.

Another symptom is overeating and a craving for carbohydrates. Weight gain can occur in people with SAD.

Avoiding social activities and social withdrawal are common symptoms. People with SAD often say they feel like hibernating.

What are some of the risk factors for SAD?

Geography may be a factor. Research shows that people who live in areas farther from the equator — where there are fewer daylight hours in the winter — are at a greater risk of winter-pattern SAD.

Temperature may have an impact, as well, but it is not clear why. It could be that very low temperatures have a direct effect, or it may be that in cold weather, “we don’t go outside as much and get much light exposure,” said Ruth Benca, chair of psychiatry and behavioral medicine at Wake Forest University School of Medicine.

What is the role of sunlight in seasonal depression?

Research shows that light has a direct impact on alertness and mood, and without it, some people may be more susceptible to depression.

Our circadian rhythms — which run a roughly 24-hour cycle and tell us when to wake up, eat and go to sleep — are set by daily light exposure, Benca said.

There are also seasonal rhythms, which run the course of the year and are set by the duration of sunlight during the day, affecting things such as our appetite, sleep patterns and mood, she added.

With shorter days and lower levels of sunlight in the winter, some people may experience disruptions in the circadian clock, possibly throwing the production of important chemicals in the body — such as cortisol, the primary stress hormone; melatonin, which promotes sleep; and serotonin, which is needed for mood regulation — out of sync with the solar clock. And, for some, this may make them more vulnerable to depression, experts suggested.

Why are some people more susceptible to seasonal depression?

Sunlight is so important to our brain and body that the retinas in our eyes have specialized photoreceptors that send signals to the brain’s perihabenular nucleus, which regulates mood. These receptors also send signals to our central circadian clock, which regulates our circadian cycles in behavior and physiology, said Phyllis Zee, a neurologist and director of the Center for Circadian and Sleep Medicine at Northwestern University’s Feinberg School of Medicine.

Scientists do not know for certain why some people are more susceptible to SAD, but one theory is that they may have less-sensitive photoreceptors in their eyes, meaning they are not taking in enough sunlight to keep these clocks aligned, said Kathryn Roecklein, an associate professor of psychology at the University of Pittsburgh.

“Their photoreceptors just aren’t responding as much as people who don’t get SAD,” she said.

What are the treatments for SAD?

Phototherapy, also known as bright-light therapy, is meant to mimic the outdoor light and is a common treatment for SAD. It involves exposure to a bright-light box for a certain period of time each day, typically in the morning. Though, in some cases, medical and mental health professionals may recommend evening treatments, too.

Experts say people should not attempt light therapy without first consulting a medical or mental health professional. Rohan recommended speaking with a professional about which type of bright-light box to buy — usually one with 10,000 lux — what time of day to use it, and how often and how long to use it, which may depend on individual needs.

Other treatments include cognitive behavioral therapy, antidepressant medications and establishing healthy daily routines.

One way to cope is to set a schedule for appropriate wake and sleep times, eating, exercising and socializing with others, Benca said. It’s important for people who believe they may have SAD to get diagnosed by a medical or mental health professional and not attempt to self-diagnose the condition. That way, they can make sure to get the appropriate treatment for it.

“Since this is a disorder that is largely related to your circadian and seasonal rhythms, things you can do to keep your rhythms properly entrained certainly can’t hurt,” she said.