The lack of natural sunlight during shorter days in fall and winter is a key factor in the onset of SAD. Sunlight plays a crucial role in regulating serotonin levels, a neurotransmitter that affects mood. Reduced light exposure can also disrupt melatonin production, which influences sleep patterns. These changes can lead to symptoms such as fatigue, sadness, and difficulty concentrating. Conversely, longer days and increased sunlight in spring and summer often alleviate these symptoms for those affected by SAD.