The lighting industry continues to explore potential benefits of lighting solutions that optimize human health. Studies indicate light has physiological effects that go beyond vision.
In recent years, the industry has focused on the impact of light on the human circadian system, and how electric lighting choices thereby can affect circadian health.
The circadian system is very important. This system produces and regulates bodily functions based on 24-hour cycles, or circadian rhythms. A big example is the sleep-wake cycle. Disrupting circadian rhythms can contribute to poor nighttime sleep, increased daytime napping, and greater risk of depression, obesity, diabetes and seasonal affective disorder.
The daily change from light to dark is the primary stimulus for synchronizing circadian rhythms to our location. The human eye has cells that are receptive to light and that connect directly to the brain’s master clock, converting light into neural signals that regulate circadian rhythms. For millions of years, the sunrise-sunset cycle performed this job, but in the modern era, we rely on electric lighting systems.
Traditionally, these lighting systems are designed for vision, disregarding light’s non-visual impact. Mariana G. Figueiro, PhD, Professor and Light and Health Program Director at the Lighting Research Center at Rensselaer Polytechnic Institute, says there are four major factors in designing a lighting solution that optimizes circadian health.
First and foremost is intensity, or the quantity of light falling on the eye’s photoreceptors during the day. What’s important here is light at the plane of the cornea or eye, not light falling on the horizontal workplane. Next is spectral power distribution (SPD)—the combination of wavelengths of light being emitted by the light source. Circadian regulation is most responsive to short-wavelength light (460 nm, or “blue”). (Note SPD and correlated color temperature, or CCT, roughly correlate but not exactly; request SPD data for a light source, not just CCT.) Meanwhile, longer-wavelength light (“red”) can also produce an alerting effect. Finally, timing, duration and photic history are also important—when the light is received, how much light cumulatively falls on the eye throughout the day, and previous light exposure.