…there are specific photoreceptors in the eye that only respond to changes in light and dark, and which are used…

…there are specific photoreceptors in the eye that only respond to changes in light and dark, and which are used almost exclusively to regulate our circadian rhythms. These melanopsin receptors connect directly to the part of the brain that regulates our internal body clocks. They work even in many people who are blind: though they can’t see anything else, their bodies still know how to adjust their circadian clocks to stay on schedule. 

http://www.newyorker.com/science/maria-konnikova/why-cant-we-fall-asleep


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