Light and health: What do we know about the science?
The daily pattern of light and dark falling on our retinas regulates our bodily processes and behavior by setting the timing of the brain’s biological clock, which regulates our circadian rhythms (i.e., physiological functions that run on approximate 24-hour cycles). The biological clock regulates our sleep–wake cycles, changes in core body temperature, and the secretion of hormones, to name just a few.
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Exposure to the wrong spectrum of light at the wrong time of day, such as short-wavelength (blue) light at night when we are preparing to sleep, disrupts the circadian system and can lead to adverse effects such as poor nighttime sleep, daytime sleepiness, depression, diabetes, cardiovascular disease, and even cancer.
Our modern built environment and frenetic 24-hour lifestyle are well equipped for circadian disruption. The electric lighting used in today’s modern buildings has been manufactured, designed, and specified to meet requirements of our visual systems — not our circadian systems — and typically delivers ill-timed, inappropriate levels and spectra of light for maintaining circadian health. We may be getting too little circadian effective light during the day and too much in the evenings.
Mariana G. Figueiro.
Light and health: Are we ready for applications?
The LRC’s research has established that optimizing light exposure for circadian health is crucially dependent on the quantity, spectrum, timing, and duration of that light exposure.
Our research with day-shift office workers has shown that receiving appropriately timed, high levels of circadian-effective light in the workplace can improve sleep quality, promote entrainment to the solar day, and relieve depression. Translating that increase in alertness into better performance is still tricky, however, because performance is affected by many other things that are unrelated to light exposures. Similar approaches have also been employed to improve sleep health in adolescents with delayed sleep disorder and to mitigate the symptoms of Alzheimer's disease in older adults. In fact, the science is clear when it comes to Alzheimer’s disease patients. Circadian-effective light during the day will improve sleep quality, relieve depression, and reduce agitation in this population.
But it is not just about blue light. Our research with rotating-shift healthcare workers, for example, has demonstrated that red light, which does not suppress the hormone melatonin, can increase alertness and reduce sleepiness in those forced to stay awake when the body is telling them to go to bed.
Lighting beyond the office
Even if a particular building provides ideal circadian-effective lighting, either in the form of daylight or spectrally tuned lighting, exposure to the wrong kind of light after leaving the office can still lead to circadian disruption. Lighting at home is just as important and should not be neglected. For this reason, personalized, portable, dynamic plug-in lighting products can provide an effective solution, although these types of devices are not yet widely available. The work environment might still be, however, where we have the greatest opportunity to receive the entraining light we need on a daily basis, especially in winter months, when some of us go to and return from work in darkness.
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Circadian wellbeing depends upon one’s history of light exposure over 24 hours, not just the light received in the workplace. Therefore, future research will be needed to characterize total light exposures experienced by the individuals during their waking hours. In the meantime, the successful application of light is more likely to occur in more-controlled environments, such as assisted living facilities and nursing homes.
Developing circadian metrics
Lighting for the circadian system employs lighting design objectives that differ from those typically used in traditional architectural lighting design, and therefore, requires metrics that differ from those currently used by lighting designers. We have developed a new way of quantifying light’s impact on the circadian system, called circadian stimulus (CS), and a CS calculator to help lighting professionals select light sources and light levels that will increase the potential for circadian-effective light exposure in buildings. Other metrics, such as melanopic lux, have been proposed, but further research needs to validate these metrics in the field.
To help designers implement some of these solutions, the LRC is also developing lighting patterns, available online, that will serve as tools for designers to learn how to implement lighting schemes that promote circadian entrainment in the built environment. Each lighting pattern shows the base case along with the new lighting design that is being analyzed for CS.
Not knowing all of the answers shouldn’t stop us from designing and implementing new lighting solutions targeted to deliver circadian-effective light to the built environment. While claims for improved performance cannot be made yet, no one can refute the idea that light isn’t just for vision. The design of our living spaces, therefore, shouldn’t be just for vision either.
MARIANA FIGUEIRO is program director of the Lighting Research Center and a professor at Rensselaer Polytechnic Institute, Troy, NY (lrc.rpi.edu).
