Seasonal Affective Disorder (SAD) affects around 5% of the US population, predominantly women, and can cause symptoms such as irritability, sadness, and changes in appetite and sleep. SAD is more common in northern regions with less sunlight, and it is believed that a decrease in serotonin levels and increased melatonin production due to decreased sunlight play a role in its development. Treatment options include light therapy, medication, psychotherapy, lifestyle adjustments, and personalized approaches. Low vitamin D levels have also been linked to SAD, and some individuals may benefit from vitamin D supplements, although it's important to consult a healthcare provider.