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IN 1999, AFTER ten years of careful work, a researcher at Imperial College in London named Russell Foster proved something that seemed so unlikely that most people refused to believe it. Foster found that our eyes contain a third photoreceptor cell type in addition to the well-known rods and cones. These additional receptors, known as photosensitive retinal ganglion cells, have nothing to do with vision but exist simply to detect brightness — to know when it is daytime and when night. They pass this information on to two tiny bundles of neurons within the brain, roughly the size of a pinhead, embedded in the hypothalamus and known as suprachiasmatic nuclei. These two bundles (one in each hemisphere) control our circadian rhythms. They are the body's alarm clocks. They tell us when to rise and shine and when to call it a day.

[…]

"What's really interesting about these third receptors," Foster told me when we met in his office at Brasenose College, just off the High Street, "is that they function completely independently of sight. As an experiment, we asked a lady who was completely blind — she had lost her rods and cones as a result of a genetic disease — to tell us when she thought the lights in the room were switched on or off. She told us not to be ridiculous because she couldn't see anything, but we asked her to try anyway. It turned out she was right every time. Even though she had no vision — no way of 'seeing' the light — her brain detected it with perfect fidelity at a subliminal level. She was astonished. We all were."
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(Bill Bryson, The Body: A Guide for Occupants)