PHILADELPHIA, Dec. 12, 2012 /PRNewswire/ -- Wintertime blues. Seasonal depression. As the Northern Hemisphere approaches the Winter solstice on December 21 -- the shortest day of the year -- up to 10% to 20% of the population will suffer recurrent episodes consistent with Seasonal Affective Disorder (SAD), mood swings that include low energy, irritability, weight gain, and overeating. (Photo: http://photos.prnewswire.com/prnh/20121212/PH28848 ) Climate, genetic vulnerability, and sociocultural factors all play a role in this very real mood disorder, according to Meir H. Kryger, MD, professor of Medicine at the Yale School of Medicine and editor of Principles and Practice of Sleep Medicine, 5 th Edition (Elsevier 2011) and Kryger's Sleep Medicine Review (Elsevier 2011). The shorter days and decrease in sunlight are triggers, and they can affect the levels of dopamine, serotonin, and norepinephrine that the body releases. Melatonin, a hormone secreted in the brain and produced at high levels in the dark, is believed to play a role in the development of Seasonal Affective Disorder, and phototherapy is commonly used to treat it, Kryger adds. Phototherapy presupposes that artificial light at a similar strength to natural sunlight will prevent the biologic changes that mediate SAD. "Seasonal Affective Disorder is a mood disturbance, but unlike typical symptoms of major depression, patients often experience increased appetite for carbohydrates, weight gain, and increased sleep duration," Kryger says, adding that the symptoms of SAD often recur annually. "Timed light exposure can help shift the body's circadian system to treat both sleep phase and depressive disorders. In winter depression, the optimal timing of light therapy must be specified relative to the patient's circadian clock rather than solar time. Light exposure is more effective than placebo treatment when light is administered in the evening or in the morning. However, it appears that morning exposure is more effective than evening exposure."