Why blue? Blue’s power to reset circadian rhythms is not intrinsic to the color. A photoreceptor for any color could have evolved to signal daylight to the suprachiasmatic nucleus. But the blues more easily penetrate the surface of the oceans—where life (and photoreceptors) likely first evolved—than do other visible wavelengths. The color balance of the sky may have helped to preserve blue’s clock-setting role throughout evolutionary history.