The exact cause of SAD isn't clear — it's believed to involve disruptions in our body's internal clock (circadian rhythms), reduced light sensitivity in the eyes during winter, genetic factors, changes in brain chemicals (like serotonin), and how our eyes respond to light levels.The exact cause of SAD isn't clear — it's believed to involve disruptions in our body's internal clock (circadian rhythms), reduced light sensitivity in the eyes during winter, genetic factors, changes in brain chemicals (like serotonin), and how our eyes respond to light levels.