It's not spiritual. Google some studies on depression. If it was spiritual, anti-depressant drugs wouldn't exist. But they do exist, and work. They help raise serotonin levels. Also, for someone like me that also suffers from Seasonal Affective Disorder, I know that is purely biological. I'm very sensitive to levels of light and when the days get short and there is less light, my serotonin levels drop. A lot of people suffer from this as well.