Seasonal affective disorder, or S.A.D., can be a formidable consequence of shorter days, colder weather, and weaker sunlight. However, it’s highly treatable, thanks to treatments such as light therapy. Even walks outside can help boost your mood, says Daniel Z. Lieberman, MD, professor of psychiatry and behavioral sciences and vice chair for clinical affairs at the George Washington University (GW) Medical Faculty Associates (MFA). In a recent article, he explores the cyclical nature of both human biology and Mother Nature, how our mental health can be affected, and how to effectively use light therapy.