Much as your ear does double duty (balance plus hearing), Russell Foster posits that the eye has two jobs: creating vision, but also -- as a completely separate function -- managing our perception of light and dark, providing the clues that our circadian rhythms need to regulate sleep-wake cycles. He and his team at the University of Oxford are exploring a third kind of photoreceptor in the eye: not a rod or a cone but a photosensitive retinal ganglion cell (pRGC) that detects light/dark and feeds that information to the circadian system. As Foster explains: "Embedded within our genes, and almost all life on Earth, are the instructions for a biological clock that marks the passage of approximately 24 hours." Light and dark help us synchronize this inner clock with the outside world.
 
The research on light perception hits home as we age -- faced with fading vision, we also risk disrupted sleep cycles, which have very serious consequences, including lack of concentration, depression and cognitive decline. The more we learn about how our eyes and bodies create our sleep cycles, the more seriously we can begin to take sleep as a therapy.