Gloomy, cloudy days and the transition from summer to winter are dreaded by many for the same reason: shorter days with less sunlight.

Internal body clocks known as circadian rhythms help us adjust our sleep and wakefulness accordingly. But for some, the disruption might lead to Seasonal Affective Disorder.

PRODUCTION CREDITS: Digital Producer: Ari Daniel Research: Fatima Husain Interview Footage: Fatima Husain and Ari Daniel Additional Animation: Ekin Akalin

Additional Visuals: Richard Lang and Minh-Thanh Nguyen Pixabay, Shutterstock, Videoblocks

Music: APM Penguin sounds: Peter Boesman, XC392706

© WGBH Educational Foundation 2018