The pineal eye?

Full Moon Could Spur Bad Sleep, Study Suggests

[W]e, like every other species on Earth, evolved on a particular planet with a particular set of astronomical cycles -- day and night, full moons and less full -- and our circadian systems adapted. It’s hard to say where the internal clock is in, say, a flowering plant, but in humans, it’s likely in the suprachiasmatic nuclei, a tiny region of the brain near the optic nerve involved in the production of melatonin, certain neurotransmitters and other time-keeping chemicals, all in a rhythm consistent with both its terrestrial and cosmic surroundings. Physically, human beings may be creatures of just this world, but our brains -- and our behavior -- appear to belong to two.

[emphasis mine]

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