How does light affect your mood?
Sunlight triggers your brain to release a chemical called serotonin. That, in turn, triggers feelings of happiness. Less sunlight may lower serotonin and cause depression.
Sunlight also helps the body make vitamin D, which boosts serotonin. Less sun and less vitamin D may cause your brain to make less serotonin.
Another brain chemical that may play a role in SAD is melatonin. When it gets dark, the brain releases melatonin to help you get to sleep. But shorter days and earlier darkness can cause the body to make more melatonin. So you can feel tired all through the day.