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A Third Type of Cell In Your Retina Regulates Circadian Rhythm

By Chad Upton | Editor

You probably learned about rods and cones in high school biology class. These retinal cells allow our brains to process light so we can see. But, there is a third type of cell that most people don’t know about: photosensitive ganglion cells.

They don’t have a cool name like Rods and Cones, but what they lack in name they make up for in swagger. Much like rods and cones, they send light information to the brain. Instead of using this information to “see”, the brain uses it to synchronize your body’s circadian rhythm to the 24-hour light/dark cycle of this planet. These “lion” cells are the original atomic clock.

They’re also used to control the pupillary light reflex. When you doctor, or local police officer, shines a bright light in your eyes, these cells are used to close your irises to limit the bright light hitting your retinas. Additionally, they help regulate melatonin — the hormone that controls several biological functions, including sleep.

Most research indicates that the ganglion cells are sensitive to light in the spectrum between 460 and 484 nm, or “blue” light which is prevalent in the visible spectrum of sunlight. Basically, this is why you get sleepy when it gets dark and why you start to “wake up” when it gets light outside.

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