In the last issue 3 (April 25, 2019), there was an introduction to "Light and Retinal Cells". The effects of light on retinal cells for circadian rhythm (or biorhythm) will be explained in this issue.
Retinal cells in the human eye receive external light and transmit visual functions (eg, color, shape) and non-visual functions (eg, circadian rhythm) to the human brain (Fig. 1). These human retinal neuronal networks have been able to recognize each wavelength from natural sunlight for over four million years and have also played an important role in the maintenance of human circadian rhythm. 

In particular, ipRGC, which are involved in regulating circadian rhythm, contains a photoreceptor protein called melanopsin. Absorption spectrum of this photoreceptor protein shows the maximum absorption wavelength around ~ 479 nm (Fig. 2). The ipRGC integrates light signals not only around ~ 479 nm through its own photoreceptor but also coming from the cone and rod retinal cells, to transmit the integrated light signals to the brain. Therefore, it indicates that the entire spectrum of natural sunlight, including blue light as well as other visible light, is important for circadian rhythm control.