A scientific consensus, based on statements from scientists studying human circadian systems, finds that circadian disruption causes ill health
The Issue
1.0 Introduction
The timing, duration, intensity and spectral composition of ocular light exposure have a profound effect on circadian clocks and rhythmic physiological processes. The human circadian system has been entrained over 10,000 generations to the contrast between the bright daylight (10,000–100,000 lux) and nocturnal darkness (0.0001–0.1 lux) of the Earth’s 24-h cycle (Moore-Ede et al., 1982). However, for less than four generations over the past century, the natural 24-h cycle of daylight and darkness at night has been replaced in the developed world by electric light. Approximately 90% of our time is now spent indoors (Kleipeis et al., 2001) under electric light, that is, typically 100 times dimmer during the day than natural daylight and 100 times brighter after dusk than even the brightest moonlight.
The resulting circadian disruption–that is to say, the out-of-sync timing of our physiological and biochemical processes that occurs in the absence of robust circadian entrainment by light–has been studied extensively over the past 50 years (Moore-Ede et al., 1982; Fishbein et al., 2021). By 2007, there was sufficient evidence for the World Health Organization (WHO) International Agency for Research on Cancer (IARC) (Straif et al., 2007) to classify night shift work with circadian disruption as a probable (group 2 A) human carcinogen based on human epidemiological studies and research with animal models. Since then, a large number of studies have linked circadian disruption caused by inadequate exposure to light during daytime hours and exposure to electric light at night to a wide range of health disorders, including obesity, diabetes, heart disease, reproductive and psychiatric disorders, and certain endocrine-sensitive cancers, such as breast cancer (Bedrosian et al., 2016; Blask et al., 2011; Fishbein et al., 2021; Kim et al., 2023; Rüger and Scheer 2009; Stevens et al., 2013; Straif et al., 2007). The circadian clocks that regulate the timing of physiological processes, and the molecular mechanisms that control them as defined by 2017 Nobel Prize in Physiology or Medicine winners Jeffrey Hall, Michael Rosbash and Michael Young (Van Laake et al., 2018), are uniquely sensitive to the blue content of visible light.
The obvious solution is to provide blue-rich light during the daytime to help entrain circadian rhythms and blue-depleted light during the evening hours to protect against circadian disruption. However, the $130 billion/year lighting industry, driven by energy- efficiency incentives, offers mostly unmodulated blue-chip LED light. Less than 0.5% (0.26/130 billion US dollars) of the lights sold today have any circadian supportive properties (Data Bridge Market Research 2023). Recognizing the importance of balancing energy efficiency and human health concerns, we sought to determine if a scientific consensus can be reached between currently active scientists, which would 1) clarify the core well-established findings of circadian lighting science, 2) provide direction to the lighting industry on the properties of the lights they should be manufacturing and marketing to promote health and wellbeing, 3) provide guidance to lighting consumers on the evidence-based characteristics of the lights they should be buying, and 4) provide the scientific community with guidance on unresolvedquestions and issues that should be prioritized in future scientific investigations.
1.1 Survey Design
Forty statements were developed for testing by the authors of this article. Thirty were potential factual conclusions summarizing the scientific literature with the following response options: 1) Don’t Know, 2) No Evidence, 3) Limited Evidence, 4) Good Evidence or 5) Well-Established. Five were potential practical advice conclusions about lighting with the same set of response options. The five remaining statements were potential expert policy statements based on the scientific literature with the following response options:
1) Don’t Know, 2) Strongly Disagree, 3) Disagree, 4) Agree or 5) Strongly Agree.
1.2 Definition of Consensus
In a large and diverse group of scientists addressing complex scientific questions, achieving unanimous agreement is not feasible or expected. In Delphi health policy consensus studies, where there are several rounds of feedback between participants enabling participants to change their minds and refine their answers based on new data and the judgments of their peers, 70%–75% agreement is often accepted as consensus (Barrios et al., 2021; de Raaff et al., 2017). In this single iteration survey, with no opportunity to revise answers based on feedback from other participants, we defined a consensus as when two-thirds of the respondents (66.7%) supported the statement. For evidence-based statements and practical advice statements, responses of “Good Evidence” and “Well-Established” were combined; for policy opinions, statements of “Agree” and “Strongly Agree” were combined.
2. Results
The ranking of support for every scientific statement is displayed in the bar graph in Figure 1 (top), and the ranking of the expert opinions on practical applications in Figure 1 (bottom), with the red line indicating the 2/3 consensus threshold.
The first set of consensus statements related to the role of regular exposure to bright light during the day (daylight or bright indoor electric light) in enhancing the robust entrainment of circadian rhythms and maintaining health. There was strong consensus that robust circadian rhythms are important for health (95.1%) and that disrupting circadian rhythms can cause ill health (98.4%).
There was also consensus that increasing daytime light intensity indoors within the normal indoor intensity range of 50–500 lux enhances circadian entrainment and strengthens circadian rhythms (70.5%), improves daytime alertness and reduces sleepiness (74.7%).
The second set of consensus statements related to the impact of electric light at night. There was strong consensus that increasing indoor light intensity at night increases the disruption of circadian rhythms (90.6%) and increases the suppression of nocturnal melatonin production (94.6%). There was also consensus that repetitive and prolonged exposure to nighttime light bright enough to cause circadian disruption increases the risk of breast cancer in women (67.6%), obesity and diabetes (74.7%), and sleep disorders (87.4%).
The third set of consensus statements related to the impact of the blue wavelengths present within white illumination on the human circadian system. There was strong consensus that “the sensitivity peak of the ipRGC melanopic receptors in the human retina is approximately 480 nm in the blue part of the visible spectrum” (97.2%) and that “the most potent wavelengths for circadian entrainment are 460–495 nm blue light near to the sensitivity peak of the ipRGC melanopic receptors” (92.7%).
The fourth set of consensus statements related to the practical application of circadian science to lighting. There was consensus that “light used in the evening (during the 3 hours before bedtime) should have as little blue content as practically possible” (82.5%) and that “the risk of circadian disruption during the 3 hours before bedtime can be reduced either by 1) dimming indoor lighting which may compromise the ability to perform visual work tasks, or 2) reducing the blue content of indoor lighting maintained at the intensity required for visual tasks” (72.0%).
The participants were also asked their expert opinion about the implications of circadian science for the design and implementation of lighting. There was consensus that “the blue content of light entering the eyes is much more important in determining circadian health outcomes than the correlated color temperature (CCT) of the light source” (86.7%) and “increasing the energy efficiency of lights is desirable, but not if it increases the risks of causing circadian disruption and serious illness (93.2%).
Summarizing their expert opinion, there was consensus that “there is now sufficient evidence to support the widespread introduction of circadian lighting that adjusts light intensity and blue content across day and night to maintain robust circadian entrainment and health (85.9%), and “LED lights with high 460–495 nm blue content should carry the warning label “maybe harmful if used at night” (79.1%). Respondents also reached consensus that “there is significant variation in individual sensitivity to light, therefore circadian lighting should be optimized where possible using personalized solutions” (90.6%).
Ranking of level of support for each statement. Top: Scientific statements. Bottom: Opinions on practical applications. Statement support was based on the combination of the response options “well established” and “good evidence” (scientific statements), and on the combination of the response options “strongly agree” and agree” (opinions). Vertical red line = 66.6% consensus level. LAN = Light at Night.
