Variations in weather patterns, such as decreased sunlight during rainy seasons, are often associated with mood changes like sadness, while sunny days tend to promote happiness. When these seasonal mood variations are severe and clinically diagnosed, they fall under Seasonal Affective Disorder (SAD). Serotonin, a neurotransmitter involved in mood regulation, mediates these changes, demonstrating a relationship between normal seasonal variations in sunlight and brain serotonin levels.