Fig. 1.

A) The human retina is made up of an outer layer of rod and cone photoreceptors (outer nuclear layer, ONL), a layer of bipolar cells (as well as horizontal cells and amacrine cells) in the inner nuclear layer, and a layer of RGCs. Cones and rods signal through bipolar cells which then make synaptic connections with RGCs at various levels of an IPL. In humans with trichromatic vision, there are three types of cones: long wavelength-sensitive (L-cone), middle-wavelength sensitive (M-cone), and short-wavelength sensitive (S-cone). RGCs are the cells which carry light information to the brain. Melanopsin-containing ipRGCs (represented in light blue) can be directly activated by light and send information to the brain’s central circadian clock. B) St Hilaire et al. show that in the early stages of a long-duration light exposure, cones (particularly S-cones) contribute significantly to the circadian response to light in human subjects. This is in addition to activation of ipRGCs by melanopsin. C) In prolonged exposure to blue light, cone activity subsides, and the response is dominated by melanopsin activation in the ipRGCs.