Our biological clocks, like those of other vertebrates, keep time thanks to the response to sunlight of the pineal gland. Stuck underneath our cerebral hemispheres, the lentil‐sized gland can't detect light directly. Rather, it receives signals from our eyes via a tortuous circuit of nerve cells. Whatever pineal photosensitivity our ancestors once enjoyed lost out to a higher evolutionary priority: boosting the computing power of the mammalian brain.