Why do leaves change color in the fall? 🍁Here's the science behind the vibrant fall colors and what triggers leaf color changes! The main reason for the eye-popping color change is not autumn’s chilly weather but sunlight. . .or rather, the lack of daylight. Day and night are roughly equal in length on the autumnal equinox in late September, but afterward, nights are growing longer and days shorter. As the autumn days shrink, the reduced daylight tells deciduous plants that it’s time to stop ga