What Is Seasonal Depression? Seasonal depression, also known as Seasonal Affective Disorder (SAD), follows a predictable seasonal pattern. It usually appears in late fall or early winter when daylight drops, and your internal system responds to the shift. This isn't just about feeling moody because it’s cold outside. Your brain and body react to changes in sunlight, serotonin, melatonin, and circadian rhythms. In other words, your system is not broken; it is responding. Commo