Every living thing is regulated by a roughly 24-hour internal clock, which keeps ticking even when there is no light around to signal what time it is. But here’s where things get tricky. There isn’t just one body clock – groups of molecules have their own individual circadian rhythms that are regulated by a single master timekeeper in the brain, known as the suprachiasmatic nucleus, or SCN. One of the more important discoveries that Foster’s research group has made is that specialized light-sensitive cells in the eye feed information to the brain and help regulate the SCN. The discovery has helped scientists understand not only how light affects whether we feel sleepy or alert, but also why crossing time zones gives us jet lag. They’ve also found links between severe mental illness and out-of-kilter body clocks, suggesting that one way to help such sufferers is simply to help them reset those clocks.