When the warm, sunny days of summer fade slowly into long, cold winter nights, it’s easy to start feeling a little down. No more carefree weekends at the beach, cookouts with the family, or lying by the pool. The days are shorter, the nights are cooler, and the weather is unforgiving (especially in New England).

But for some, feeling a little bummed after summer is more than just disappointment that their tans are fading. They feel severely depressed, anxious, lethargic, irritable and sad during the winter months year after year.

According to WebMD.com, these are all symptoms of a condition known as seasonal affective disorder (SAD).

SAD is a form of depression that affects people at the same time every year, generally between the months of September and May. During these months, the days are shorter, limiting the amount of sunlight you are exposed to.

This lack of light leads to a lowered production of the brain chemical serotonin. Jean Lawrence of WebMD explains that serotonin is “a mood-enhancing chemical that regulates hunger and the feeling of well-being.” Lawrence writes that the brain’s production of serotonin increases with light exposure.

So when the sun goes down before the evening commute home, the darkness in the skies brings a dark cloud over your mind.

Other causes of the disorder, such as problems with serotonin regulation and the body’s biological clock are also being researched. Symptoms can come on either suddenly or gradually.

“What this means for college students is less social activity, more isolation, decreased motivation for schoolwork, mild to moderate sadness, and a general lack of interest in things,” said Patricia Challan, assistant director of Emerson College’s Counseling Center. “Especially for Emerson students, who are often involved in multiple activities beyond the classroom, experiencing seasonal depression can take its toll.”

So how can you be sure if your routine winter blues are really the sign of a more serious mental condition? Doctors use questionnaires and mental health assessments to diagnose SAD after a patient has shown consistent symptoms for at least two years in a row.

“I think it’s most important that people understand that many of us may experience ‘the winter blues,’ but most will not exhibit full-blown seasonal affective disorder. The difference is in intensity and number of symptoms,” Challan said.

According to WebMD, between 60 and 90 percent of people with SAD are women.

While females between the ages of 15-55 are the most at risk for SAD, the disorder is uncommon in people under 20.

In an article on the Mayo Clinic’s Web site, there is also such a thing as reverse SAD. This form of SAD is very rare, and has opposite symptoms during the opposite time of year. People with reverse SAD experience periods of mania during the summer months.

Antidepressant medications such as Prozac and Zoloft are often prescribed to patients, but their labels warn of side effects that mimic their original symptoms: loss of appetite, nausea, drowsiness and difficulty sleeping. Counseling, whether alone or in a group, can be effective in the treatment of SAD because it helps sufferers understand their disorder, as well as teaching them ways to cope with it.

Light therapy is another popular treatment of the disorder. Because a lack of light inhibits the brain’s production of serotonin, routine exposure to bright lights during dreary winter days can help those afflicted to feel better when they’re down. WebMD describes a form of light therapy known as “dawn simulation,” in which lights are electronically programmed to turn on slowly in the hours before you wake up in the morning.

Self treatment for SAD is also recommended. Lawrence advises people to turn on bright lights indoors, open blinds and shades in the windows, eat wisely, and exercise regularly. The Mayo Clinic adds that you should spend time outside and try to manage stress levels as well.

The Seasonal Affective Disorder Association (SADA) in England is the largest support organization for the disorder, with over 2,000 members. SADA is a registered charity that spreads awareness and information about the disorder, releases a widely distributed newsletter, and raises money for SAD research.

The organization also serves as a support group for those affected by SAD. On its Web site (sada.org.uk), it offers a yearly membership for about $22, which includes the group’s Information Pack ($10 if purchased separately), newsletters, and updates on research, treatment options, upcoming events, and reading materials.

Mental Health America (MHA) acknowledges a more specific form of SAD: holiday depression. Although the holidays are supposed to bring feelings of joy and happiness, the stress of money, guests, and shopping causes some people to dread the so-called “happiest season of all.” MHA points to stress, fatigue, unrealistic expectations, financial troubles, and family pressures as causes of holiday depression. To control the tension and keep your holidays as positive as possible, MHA advises you to properly budget both your time and money, spend time with the people you love, do something for others (such as volunteering at a homeless shelter), allow yourself some quality time for rest to “recharge your batteries,” and avoid excessive drinking.

“Some students may have a tendency to use substances more when they are depressed, which usually worsens the symptoms overall. Alcohol, in particular, is a depressant, and can worsen depressive symptoms,” Challan said.

So now that the sun is going down long before dinnertime and the temperatures are starting to drop, try not to let the winter blues keep you holed up in your bedroom, dreading the months to come. Flip on those desk lamps, go for a brisk walk around the Common, grab an apple instead of the potato chips, or try doing yoga. And, as Challan recommended, “Remember that winter is a season, with a fixed beginning and an end.”

Winter blues got you down? Places to go if you need some help.

Emerson College’s Counseling Center (617) 824-8595

Depression and Related Affective Disorders Association (410) 583-2919

Substance Abuse and Mental Health Services (800) 789-2647

American Psychological Association (202) 336-5500

National Institute of Mental Health (301) 443-4513

National Women’s Health Information Center (800) 994-9662