The structure of the Limulus polyphemus retina changes with the time of day. The structural changes exhibit a circadian rhythm when the animal is kept in constant darkness. During subjective night, the aperture of each visual unit (ommatidium) widens and flattens, allowing more photons to impinge on the underlying photoreceptors. In addition, the photosensitive region (rhabdom) of the photoreceptors widens and shortens. During subjective day, these changes reverse: both the aperture and rhabdom elongate and narrow, reducing the number of photons caught by the photoreceptors. The endogenous structural rhythm is mediated by efferent optic nerve activity generated from a circadian clock located in the brain. Eliminating the efferent input by cutting the optic nerve blocks the rhythm and shocking the cut nerve restores it. Natural lighting magnifies the structural rhythm. Daylight induces photomechanical movements, enhancing the endogenous daytime changes that decrease photon catch. The onset of darkness enhances endogenous nighttime changes that increase photon catch. Cutting the optic nerve blocks all structural changes, both endogenous and photomechanical. In the animal's natural environment, the effects of circadian efferent input and natural lighting are thus reciprocal: The evening onset of efferent activity pushes retinal structure in one direction; the morning onset of daylight pulls it in the other. The 2 processes, however, are not equivalent. In the absence of cyclic lighting, circadian efferent input produces an attenuated structural rhythm, but cyclic lighting alone produces no rhythm.