EXPLORE THE RI ADVENT CALENDAR: http://rigb.org.uk/advent Our biological clock -- "the day within" -- regulates almost every aspect of our physiology and behaviour, but it doesn't always work on a 24 hour cycle, so our bodies have to constantly adjust to environmental time. Exposure to the light/dark cycle is one of the most important determining factors, without it our internal clock becomes out of sync with the external world. Prof Russell Foster from the University of Oxford explains how studying the response of mice with retinal diseases to a light/dark cycle lead to the discovery of a previously unknown light receptor in the eye: Melanopsin, or OPN4 genes, found on chromosome ten. It is hoped that future research on these light sensitive molecules could open up new opportunities to help people with certain kinds of visual impairments. With thanks to BBSRC: http://bbsrc.ac.uk/ Don't miss the 2013 CHRISTMAS LECTURES on BBC Four and BBC iPlayer: http://rigb.org/christmas-lectures The Ri is on Twitter: http://twitter.com/ri_science and Facebook: http://www.facebook.com/royalinstitution and Tumblr: http://ri-science.tumblr.com/ Our editorial policy: http://www.rigb.org/home/editorial-policy Subscribe for the latest science videos: http://bit.ly/RiNewsletter