Retina-clock relations dictate nocturnal to diurnal behaviors

Abstract

One hypothesis for the evolutionary expansion of mammals after their emergence from reptiles is that mammals became nocturnal and were thus able to use an unexploited temporal niche (1, 2). To produce an exact 24-h rhythm in behavioral outputs, the circadian clock develops a temporal phase relation with the day/night cycle. In species studied thus far, the electrical and gene expression rhythms in the circadian clock in both diurnal and nocturnal animals have similar phases with respect to the light:dark cycle. This suggests that the phase difference in the activity of nocturnal and diurnal animals is not caused by differences in the circadian clock but rather by how the clock couples to output mechanisms (3). In this issue of PNAS, Doyle et al. (4) present a possible mechanism for how retinal mutations could also lead to a switch between nocturnal and diurnal behavior.