Dr. Michael Terman’s CNN.com interview

 "Our inner clock, whose timing cycle does not necessarily match day and night outdoors, needs to stay in sync with rest-activity cycles dictated by family and work life," says [Dr.] Michael Terman, a professor of clinical psychology in psychiatry at Columbia University's College of Physicians and Surgeons. "This is both very disorienting and a trigger for mood slumps and depression."
http://www.cnn.com/2015/02/13/health/how-to-beat-the-winter-blues/index.html