Information content of downwelling skylight for non-imaging visual systems

Light-sensitive proteins (opsins) are expressed in non-imaging tissues like the brain, dermis and reproductive organs of most animals. Such tissues have been shown to sense the intensity and spectrum of light over time. Functional links to circadian and reproductive rhythms have been speculated but remain uncertain. Here we use information theory to quantify the natural scene for non-imaging opsins, i.e., spectral patterns in downwelling skylight. Our approach synthesizes measurements of natural downwelling spectra, atmospheric distortions, and weather, with the biophysical constraints of opsins and biochemical clocks, while minimizing assumptions about how organisms process such information. We find that tissues expressing multiple opsins could use twilight to extract significant information about lunar phase and time of day in many climates. In contrast, information in light intensity is far less robust to atmospheric perturbations. Thus our work quantifies circalunar and circadian regularities in the spectrum of downwelling radiance salient to non-imaging opsins.