Light for the Circadian System: Beans or Chili?

By Mark Rea

Everyone must eat, and beans will largely serve the purpose of filling one up. But, to enjoy eating beans, there is nothing quite like a bowl of chili. By including tomatoes, meat, cheese and, of course, proper seasoning, beans can be a fantastic meal. Beans alone, not so much.

From specifiers to scientists, everyone in the lighting community should be promoting circadian entrainment with light for occupants in schools, offices, nursing homes, and factories – anywhere people are active during the day and asleep at night. High photopic light levels (E >500 lx), high levels of equivalent melanopic lux (EML >240), high levels of melanopic equivalent daylight illuminance (mEDI > 250), or high levels of circadian stimulus (CS > 0.3) will, with an extended duration of exposure, all meet that professional obligation. But, it’s surprising that many lighting specifiers and researchers who are interested in providing circadian-effective light seem to neglect the underlying mechanisms of circadian phototransduction by the retina, essentially preferring beans to chili. Granted, it’s a whole lot easier to boil beans than it is to prepare chili, but a little culinary effort can make a big difference.

It’s also a whole lot easier to believe a single photoreceptor, the intrinsically photosensitive retinal ganglion cell (ipRGC), drives the suprachiasmatic nuclei (SCN), but what a big difference a little review of the scientific literature can provide. The evidence is clear. Alone, the workings of the ipRGCs, which contain the photopigment melanopsin, cannot explain many of the important aspects of the retinal conversion from photons to neural signals reaching the biological clock in the SCN. This single photoreceptor, while essential for circadian phototransduction, cannot explain the spectral sensitivity of the system, its operating characteristics from threshold to saturation, or the curious phenomenon of subadditivity. Every neuron type in the retina — photoreceptor, horizontal, bipolar, amacrine, and ganglion — affects the retinal conversion from photons to neural signals. The CS computational model is a recipe for circadian phototransduction that accounts for all of these essential ingredients, including the ipRGC beans.

Some have argued that for practical purposes, we only need to worry about the ipRGCs with no further consideration of the retina’s neuroanatomical or neurophysiological features. Sure, for practical purposes beans will sate one’s hunger, and high photopic illuminance levels will provide circadian-effective light to building occupants during the day. But if all we care about is beans, we don’t even need another metric like EML or mEDI to meet our professional obligation since photopic illuminance will do just as well. If, however, we care about a more toothsome bowl of chili, we need to embrace the complexity of the retina so that we can both meet our professional obligation and provide the foundation for more sophisticated and lasting lighting designs and product innovations.

One of my favorite quotes, attributed to Kodak founder George Eastman, is: “By understanding the process, you can control the alternatives.” The flip side of that quote, as it relates to circadian effective lighting, is: “By not understanding circadian phototransduction, you have limited control of the alternatives.”

A recent study found that street lighting as practiced in the United States does not suppress melatonin at night. By understanding how the human retina sets a high threshold for activation (unlike nocturnal rodents), we were able to predict this finding a decade before that experiment. As another example, a recent study showed that S-cones contribute to human nocturnal melatonin suppression. Two decades ago, we concluded that the spectral sensitivity of nocturnal melatonin suppression could only be explained if S-cones were involved. And this understanding also implied, as we later showed through our demonstration of subadditivity in three different experiments, that the S-cone participates in circadian phototransduction via the spectral opponent blue versus yellow color mechanism in the retina. If one assumes that ipRGCs are the only photoreceptor needed for circadian phototransduction, none of those insights, neither scientific nor practical, can be predicted.

Clearly, we do not understand all that can be learned about circadian phototransduction by the retina. But, isn’t it obviously better for lighting professionals and researchers to begin understanding how circadian phototransduction by the retina takes place? Surely, it’s worth the extra effort to make and serve chili. Otherwise, the result is only worth a hill of beans. ■