Light is a key driver for synchronizing our internal, circadian rhythms to the external light and dark cycle. In my last blog post, you learned about how our eyes not only enable us to see the world (through the cones and the rods), but that they also signal whether it is light or dark around us. This information, first received by a special set of cells in the eye which contain a light-sensitive protein called melanopsin, is passed onto the tiny structure in our brain that contains our inner clock: the suprachiasmatic nucleus. In most cases, light-dark cycles are defined by the availability of daylight. With the invention of artificial light, light-dark cycles can be modified as light can be switched on and off at any time.
Many lighting manufacturers have jumped on the recent scientific insight that there is an additional dimension to light, coining the buzzword “Human Centric Lighting”, to describe lighting solutions that are promoted to be ‘optimized’ in some way to be friendly to our circadian clock. But this term is ill-defined: What one company might market as “Human Centric Lighting” could be very different from what another one markets as “Human Centric Lighting”. Some lighting systems tune light according to time of day, using whitish light during the day, and removing the blue components in the evening, making it more orange. The natural illumination cycle, however, is quite different from this, with a boost of blue light during dusk and dawn when the sun is below the horizon.
It is important to keep in mind that what a light looks like is in principle separated from its effect on our body clock. Scientists have known for a long time that different light sources can appear the same to the naked eye, even when their physical spectra — which tell us how many photons each light source emits per wavelength — are different. The same white-appearing light can be made with different “light-making” technologies. A old-time CRT monitor can produce a white that looks exactly the same as a white reproduced on the most recent iPhone, even though the underlying physical spectra are quite different. Pairs of lights that look the same are also called metamers, or metameric lights. Metamers produce the same excitation in the cones, thereby appearing the same, but excite the melanopsin photoreceptor differently. Researchers, including myself, now use metameric lights to understand how manipulating the way melanopsin is activated can affect our bodies and brains differently. In certain circumstances, metameric lights can be exploited to minimize the effect of light on the body clock, while preserving appearance.
More recently, it has become clear that the biology underlying our response to light is fascinatingly complex. A recent study by researchers at the University of Manchester found that changes in light that only affect the cones in the mouse eye can affect circadian rhythms independent of melanopsin. In my own work with human participants, I found that signals from the short-wavelength cones do not cause any differences in melatonin secretion, which tells us that the timing of the body clock is not affected. What does this seeming contradiction tell us? In research, we are still just scratching the surface of how light affects our body clock, and there remains a lot more to research. In practice, we can follow two simple principles to keep our body clock healthy: Bright light during the day, dim or no light in the evening and at night. This simple strategy may be more effective than many of the commercial “Human Centric Lighting” designs out there.
About the author: Manuel Spitschan, PhD, is a researcher at the Department of Experimental Psychology at the University of Oxford (UK) and the Centre for Chronobiology in Basel (Switzerland). You can find him on Twitter (@mspitschan) and ORCID (0000-0002-8572-9268).