we used to believe that natural light sets our circadian rhythm by affecting the eyes – which are full of photoreceptors. a 1998 study shows that the skin also serves to help set our biological clocks: scientists at cornell put a wearable fiberoptic brace on people’s knees under a blanket, and illuminated the skin on the back of their knees without them knowing. people wearing the brace responded as if they were exposed to sunlight, which might point the way to re-set the biological clock artificially while in the dark. it might also explain why laying in the sun always makes me so sleepy… the researchers also set up a company to sell light therapy products called apollo health.