Light illuminates our surroundings and ensures that we can see fine details, recognize faces, discriminate ripe blackberries from those that need a little bit more time, and many other things that we use vision for. At a basic level, vision begins with the capture of photons (the fundamental electromagnetic particles of light) by the retina, a fine layer of cells in the back of our eyes. These cells are specialized to different kinds of light. We have three types of cone cells that are responsive for different parts of the visible spectrum of light, also called wavelengths. In addition to the cones, which are responsible for vision of color, motion and spatial detail in daylight, we have the rod cells. The rods are much more sensitive than the cones, providing us with a basic view of the world at night. When a photon is captured by these photoreceptor cells, it triggers a complex series of reactions, turning light in the external world into a neural signal inside our brains. This process is very complex. Neurobiologists, psychophysicists and vision scientists are continuing to study, examine and understand how the complex pattern of light striking our eyes is turned into our visual reality.
Until around 20 years ago, it was thought that the cones and rods are the only photoreceptors in the human eye. This orthodoxy was revised by the discovery of the so-called intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells are separate from the cones and rods, which are specialized to ensure that light will trigger a response. In the ipRGCs, a protein called melanopsin that is sensitive to short-wavelength or blue light is responsible for signaling how bright it is and more importantly, whether it is night or day. The ipRGCs are connected to a structure in the brain called the suprachiasmatic nucleus, which is a small structure the size of a grain of rice coordinating the timing of our brain and body. Even though the ipRGCs are sensitive to blue light, this does not mean that blue light is the only type of light that can affect our body clock. In the real world, most lights that we encounter are what we call “broadband” — they contain light of all wavelengths. If they are bright enough, all lights can affect our body clock in some way. Research on the effects of light on our brain and body is very much ongoing and we have just begun to scratch the surface. This is what makes this research field so exciting, as there is great opportunity to integrate what we know about how we see with what we know about the effects of light on the clock.
Manuel Spitschan PhD is a member of the Board of Advisors of the Center for Environmental Therapeutics, and is a University Research Lecturer and Sir Henry Wellcome Fellow at the Department of Experimental Psychology, University of Oxford. His research focuses on the effects of light and lighting on human physiology and wellbeing, with a specific interest in integrating knowledge from visual and circadian neuroscience.