The most common type of SAD is called winter-onset depression, with symptoms running from late fall or early winter through summer. (A much rarer version of SAD, called summer-onset depression, runs from late spring through winter.)

While the symptoms of SAD are very similar to the signs of clinical depression, in order to be diagnosed with winter-onset SAD, a patient must display symptoms at least two consecutive years in a row, as well as experience significant improvements in mental health and mood once the weather turns warmer, according to licensed clinical social worker Lisa Brateman. SAD is more common in women than men, and usually develops in people after the age of 20.

What Causes SAD?

While the specific cause is unknown, as its name suggests, seasonal affective disorder seems to be related to the changes in daylight during different times of the year. A massive review of SAD conducted by the Netherlands’ University Hospital found a significant positive correlation between latitude and prevalence of SAD in North America. “The incidence of SAD is more common in people who are living farther away from the equator,” Brateman says, where wintertime daylight is shorter.

Reduction in daylight exposure can cause changes in the brain chemicals serotonin, which helps regulate mood, and melatonin, which helps regulate mood and sleep patterns.

But, experts believe, there may be other factors at play. According to a survey conducted by the National Alliance of Mental Illness, 55% of patients with SAD report having at least one close relative diagnosed with clinical depression.

Additionally, a recent University of Virginia study suggests the disorder may actually be linked to a genetic mutation of the eye that renders a SAD patient less responsive to light. Individuals [with this mutation] may require brighter light levels to maintain normal functioning during the winter months, according to findings reported by  Professor Ignacio Provencio, PhD, biology professor at University of Virginia School of Medicine, who led the study.

What are the possible treatments for SAD?

For those suffering from SAD, life can be a struggle for up to half the year. Luckily there are alternatives to waiting out the calendar. The most common of these treatments is light therapy, which revolves around the use of light boxes, which radiate a controlled amount of cool, white fluorescent or full spectrum light with a built-in screen to filter out harmful ultraviolet rays. Patients undergoing light therapy are encouraged to spend one to two hours a day in close proximity to their devices, from the first onset of symptoms until the weather — along with their mood — brightens.

Other treatment options include the ones normally prescribed for depression — antidepressant medications and cognitive behavioral therapy. Work with your doctor to determine which treatment, or combination, may be right for you.

What to expect when you have SAD

Like any form of depression, SAD is a journey, not something a patient can simply “snap out of.” Finding the right treatment path can be a lengthy process that requires patience. That’s why, Brateman says, the importance of a solid support system cannot be overemphasized. “I felt like my life would never go back to how it was,” Danielle says of her initial bout with SAD. “But my family and friends stood by me.” Five years later, Danielle still identifies with her diagnosis, but doesn’t dread December the way she used to. Starting when the weather turns, she works with a therapist to process her feelings and commit to winter goals like daily swims in a heated pool, and a weekly date with friends. On especially difficult days, she uses her light box. 

“I wish that me at seventeen could see me now,” Danielle says with her trademark smile — that now comes out year round.