I first learned how our eyes work in a college neuroscience class in the fall of 2002. My textbook showed colorful cartoons of the retina, lined with two types of cells that convert light waves into the electrical currency of the brain. There were rod cells, which are useful in low-light situations, and there were cone cells, which allow us to see colors. Neat and tidy.

Unbeknownst to me (because the information hadn’t made it into textbooks yet) neuroscientists had just discovered a third class of light-sensitive eye cells, called, rather uncreatively, ‘intrinsically photosensitive retinal ganglion cells’ (ipRGCs). These cells have nothing to do with vision. They absorb light in order to properly set our circadian clock.

In the decade