For many people, shorter days and less sunlight can lead to seasonal affective disorder, or SAD. Also called seasonal depression, WebMD is defined as a mood disorder that occurs around the same time each year. It usually begins in fall or winter and ends in spring or early summer.

Courtney Shaw, a nurse practitioner at Aspirus St. Luke’s Care Center in Duluth, says that while anyone can be affected, research indicates that SAD is more common in northern states.

The Yale School of Medicine agrees, saying that SAD is more common in latitudes farther from the equator. In the United States, SAD is rare in Florida, more common in the mid-Atlantic states, and more common in the northern United States or Canada. A study using questionnaires estimated an incidence of SAD at 1% in Florida, approximately 5% at the latitude of New York, and approximately 10% at the latitude of New Hampshire. Some patients report that they did not have problems with winter when they lived in southern latitudes, but experienced significant seasonal symptoms when they moved further north.

The American Psychiatric Association (APA) reports that about 5% of adults in the United States have SAD, and it typically lasts about 40% of the year. If 5% seems low, keep in mind that SAD is more prevalent in northern states.

“The most difficult months for people with SAD in the United States are usually January and February,” declares the APA.

According to the National Institute of Mental Health (NIH), studies indicate that people with winter SAD have reduced levels of serotonin, a brain chemical that helps regulate mood. Research also suggests that sunlight affects levels of molecules that help maintain normal serotonin levels. Shorter daylight hours can prevent these molecules from working properly, contributing to lower serotonin levels in winter.

Symptoms of SAD may include:

Sad, anxious or persistent “empty” mood most of the day, almost every day, for at least 2 weeks

• Feelings of hopelessness or pessimism

• Feelings of irritability, frustration or agitation

• Feelings of guilt, worthlessness or helplessness

• Loss of interest or pleasure in hobbies and activities

• Decreased energy, fatigue or feeling slowed down

• Difficulty concentrating, remembering, or making decisions

• Changes in sleep or appetite or unexpected weight changes

• Physical pain, headaches, cramps, or digestive problems that do not have a clear physical cause and do not go away with treatment.

• Thoughts of death, suicide or suicide attempts

For winter SAD, additional symptoms may include:

• Too much sleep (hypersomnia)

• Overeating, especially with a craving for carbohydrates, leading to weight gain

• Social withdrawal (feeling of “in hibernation”)

Although not everyone with SAD experiences all symptoms, they can range from mild to severe.

There are treatment options to reduce or even eliminate the symptoms of SAD. The APA reports that SAD can be effectively treated in several ways, including light therapy, antidepressants, talk therapy, or a combination of these. Although symptoms usually improve on their own with the change of season, they may improve more quickly with treatment, especially if they have become severe.

Light therapy involves sitting in front of a light therapy box that emits very bright light (and filters out harmful ultraviolet (UV) rays). This typically requires 20 minutes or more per day, usually early in the morning, during the winter months. Most people notice improvements from light therapy within one or two weeks after starting treatment. To maintain the benefits and prevent relapse, treatment is generally continued throughout the winter. Due to the anticipated return of symptoms in late fall, some people may begin light therapy in early fall to prevent symptoms.

The NHS says that many people with winter SAD are deficient in vitamin D. Vitamin D supplements can help improve symptoms. However, the NHS warns, studies testing vitamin D as a treatment for SAD have produced mixed results, with some studies indicating it is as effective as light therapy and other studies finding no effect.

NHS UK, the United Kingdom’s government-funded health system, also recommends getting as much natural sunlight as possible and doing physical exercise.

If symptoms do not improve or begin to increase, experts recommend talking to a healthcare professional, who can properly determine whether the symptoms meet the criteria for SAD.