“… SAD is a recurrent major depressive disorder with a seasonal pattern usually beginning in fall and continuing into winter months. A subsyndromal type of SAD, or S-SAD, is commonly known as ‘winter blues.’ Less often, SAD causes depression in the spring or early summer.
The fact that SAD occurs when the days begin to darken and sunlight is at a minimum is not a coincidence. Your health and mood are intricately tied to exposure to sunlight. For example, your serotonin levels (the hormone typically associated with elevating your mood) rise when you’re exposed to bright light.