Evidence-based public health messaging on the non-visual effects of ocular light exposure: a modified Delphi expert consensus

Abstract

Introduction

In addition to vision, light regulates circadian rhythms, sleep, mood and alertness. Despite growing scientific understanding, there remains a gap in translating this knowledge into accessible, evidence-based guidance. The goal of this paper is to formulate scientifically grounded statements about the influence of light on human psychological and physiological health, intended for dissemination to the public and policymakers.

Methods

An international consortium of 21 experts convened at the Ladenburg Roundtable in April 2024. Experts were selected based on their scientific contributions to chronobiology, psychology, neuroscience, and the measurement and practical application of light. Through a structured, iterative modified Delphi process, 27 statements were developed. Each statement included a simplified public-facing version and contextual information to support understanding. Consensus was assessed using predefined thresholds of agreement (>75% endorsement). Statements not meeting consensus were revised and re-evaluated.

Results

Of the 27 proposed statements, 26 reached the threshold for consensus, with high levels of agreement across diverse topics. One statement did not reach consensus due to insufficient scientific evidence and was excluded, while another was revised based on feedback and subsequently accepted. The iterative revision process significantly improved the clarity, accuracy and accessibility of the final statements. A readability assessment showed an average sentence length of 14.8 words and a Flesch-Kincaid Grade Level of 8.6, indicating that the statements suit a broad, non-specialist audience. The final consensus statements are available at lightforpublichealth.org.

Conclusions

This expert consensus provides clear, accessible messages about how light affects human health. The statements offer a practical tool for public education and policymaking and can be used by public-health multipliers (eg, schools, employers, healthcare providers and urban planners) to promote healthier light exposure in daily life. They highlight the importance of recognising light as a key factor in health, alongside sleep, nutrition and physical activity.

WHAT IS ALREADY KNOWN ON THIS TOPIC

• Light exposure has an impact on human health beyond vision, affecting sleep, alertness and circadian rhythms. However, policy guidance on healthy lighting is limited.

WHAT THIS STUDY ADDS

• This expert consensus document sets out 26 key messages on the health impacts of light in clear, accessible language.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

• These messages provide policymakers with reliable guidance to help create healthy lighting environments. They can inform regulations, public awareness campaigns and health promotion strategies.

Introduction

Over the past 50 years, the scientific understanding of how light affects human health has advanced dramatically. Beyond enabling vision, light regulates core biological processes, including circadian rhythms, sleep and hormone secretion. The discovery of intrinsically photosensitive retinal ganglion cells (ipRGCs) revealed how light exerts effects on human physiology and psychology.1,3 These cells play a central role in synchronising the biological clock to the external light-dark cycle,^{4 5} thereby influencing sleep-wake timing.^{6 7} Additionally, light acutely affects alertness,^{8 9} melatonin production (eg,9,11) and cognitive performance.^{12 13}

These discoveries prompted the development of standards for measuring biologically effective light. In 2018, the International Commission on Illumination (Commission de l’Éclairage Internationale; CIE) introduced CIE S 026/E:2018, providing a standardised framework for quantifying biologically effective ocular light.¹⁴ In 2022, global experts released recommendations for healthy light exposure.¹⁵ However, the translation of these findings into public health guidance remains a challenge, hindered by a lag in the adoption of research findings in policy and public health guidelines.¹⁶

To address this gap, 21 international experts convened at the Ladenburg Roundtable (14–16 April 2024, Germany) to develop consensus-based, evidence-informed statements on light and health. The goal was to support public health messaging, guide policy, inform architecture and workplace standards, and empower individuals. The resulting white paper identified seven key steps for effective communication, including simplifying technical language and aligning with existing public health recommendations.¹⁷

This report presents 26 consensus statements on the effects of light on human psychology and physiology, developed through an iterative review.

Materials and Methods

Ethics

This research involved expert participants contributing in their professional capacity to a consensus process. No clinical interventions, personal health data or sensitive information were collected. Formal ethical approval was therefore not required under prevailing guidelines. All participants were informed about the purpose and procedures of the study and provided consent through their voluntary agreement to take part.

Patient and Public Involvement statement

Patients or members of the public were not involved in the design, conduct, reporting or dissemination of this research.

Participants

Participants for the Ladenburg Light & Health Roundtable were purposefully selected to ensure diversity in geography, career stage and expertise. The final group included 25 experts from human clinical research, animal neuroscience, physiological optics, epidemiology, occupational health, environmental psychology, chronobiology, lighting science and public health (online supplemental Appendix A). They represented universities, research institutions and public agencies. Four of the invited individuals were unable to attend. Of the 25 experts, 19 actively participated in the statement rating process. A coordinating committee of six lead authors representing five key organisations, the International Commission on Illumination (CIE; https://cie.co.at/), the Society for Light, Rhythms and Circadian Health (SLRCH; https://slrch.org/), the Daylight Academy (DLA; https://daylight.academy/), the Good Light Group (GLG; https://www.goodlightgroup.org/) and the Center for Environmental Therapeutics (CET; https://cet.org/), oversaw the process and endorsed the statements; they did not contribute ratings. Participants were not compensated for their participation. All invited experts completed the process.

Statement development and consensus

We employed a modified Delphi technique involving two rounds of anonymised feedback using REDCap^{18 19} (Round 1) and via email-based commenting (Round 2), with a predefined consensus threshold of 75% agreement (figure 1). The development was not registered. The coordinating committee developed the initial statements, incorporating feedback from roundtable discussions. No systematic searches were conducted. In this development stage, the committee members assessed the level of evidence using a three-alternative forced-choice procedure with three options (low evidence, ie, based on preliminary or weak findings, limited studies, or inconsistent results; moderate evidence, ie, supported by multiple studies but with some limitations, such as variability in results or study designs; high evidence, ie, strongly supported by consistent, replicated research with large sample sizes and broad scientific consensus). Committee members simplified language, added contextual information and cited relevant evidence. To minimise confirmation bias, however, all subsequent revisions were guided by anonymised ratings and open-text feedback from the broader group of expert participants, who were not members of the coordinating committee. This occurred through a two-step Delphi peer-review process: in Round 1, participants rated agreement (two-alternative forced-choice agree/disagree) and commented on the draft statements; feedback was integrated into revised versions. Participants reviewed simplified statements and contextual text in Round 2, ensuring scientific accuracy and clarity. The entire process, including consensus-building, peer review and readability adjustments, occurred between October 2024 and February 2025.

Figure 1. Flow chart. The coordinating committee led the initial development of key statements, which were then submitted for consensus approval by the Light for Public Health Consortium. Each statement received feedback, with in-depth input from four Consortium members incorporated into the revisions. Once the core statements were finalised, simplified versions and contextual information were developed, followed by an additional round of feedback. A medical writer then assessed the document for English readability, ensuring accessibility while preserving the original scientific statements. Finally, the Light for Public Health Consortium was given a final opportunity to review the scientific statements, along with their simplified versions and contextual details, before finalisation. Boxes in light blue indicate steps led by the steering committee (n=6), white presents the Consortium (n=25) and grey the independent external medical writer (n=1).

Evidence rating

Each scientific statement developed in this consensus process was rated for evidence certainty following a structured approach. We based our assessment on a modified version of the Oxford Centre for Evidence-Based Medicine (OCEBM) Quality Rating Scheme for Studies and Other Evidence.

The certainty of evidence for each statement was assigned as follows:

• Level 1 (high certainty): properly powered and conducted randomised clinical trials, or systematic reviews with meta-analysis.

• Level 2 (moderate certainty): well-designed controlled trials without randomisation, or prospective comparative cohort studies.

• Level 3 (moderate to low certainty): case-control studies or retrospective cohort studies.

• Level 4 (low certainty): case series with or without intervention, or cross-sectional studies.

• Level 5 (very low certainty): opinion of respected authorities, narrative reviews, case reports or textbook knowledge.

Readability rating

To support public understanding, a medical writer assessed readability and refined simplified statements to a high school reading level and key stakeholder groups, without altering the original scientific statements.

Reproducibility

The data supporting the findings of this study, including anonymised voting outcomes and finalised consensus statements, are available at https://github.com/tscnlab/SpitschanEtAl_BMJPublicHealth_2025 and archived on Zenodo (https://doi.org/10.5281/zenodo.17286866). This article was previously preprinted on medRxiv.²⁰ The data are licensed under the CC-BY 4.0 (Creative Commons Attribution) License, and the code is licensed under the MIT License.

Results

Initial statement development

The 27 initial statements aimed to deliver clear, evidence-informed messages about the non-image-forming effects of light. They address a wide range of topics, including the properties of light, how the human eye detects and processes light, and the influence of light on various physiological and psychological functions, such as the regulation of circadian rhythms, sleep, mood and alertness. The statements also consider how these responses may change with age and include reflections on the current state of research in the field.

A consensus threshold of over 75% agreement was met for all but two of the original statements:

“Light exposure can be described by its intensity: The total amount of energy across all wavelengths from 380 to 780 nm.”

“Higher light levels during the daytime reduce the physiological effects of light in the evening and at night.”

These statements did not meet the predefined consensus threshold, indicating variability in expert agreement regarding their formulation or interpretation (figure 2). The coordinating committee carefully reviewed all contextual feedback provided by the Consortium. Open-ended feedback revealed that the first controversial statement (“Light exposure can be described by…”) was considered too general, with multiple members recommending the inclusion of information on how the different characteristics of light are weighted. In response, the coordinating committee revised the statement accordingly to include information about the weighting function to be used (”… weighted according to the function of interest”), and it was subsequently included in the list of agreed-upon statements to be reviewed by the Consortium. In contrast, the second statement (“Higher light levels during the daytime…”) was deemed to lack sufficient scientific evidence and was therefore removed from the consensus process.

Figure 2. Consensus process and expert agreement levels. This figure illustrates the consensus results for 27 statements on the non-image-forming effects of light, displaying the proportion of participants (n=19) who agreed (green), disagreed (orange) or did not provide a response (orange). Horizontal bars display agreement levels for each statement, with darker shades indicating higher percentages. The consensus threshold is set at 75%. Statements that did not meet this threshold were either amended following contextual feedback (such as statement 3, marked with *) or excluded from further consideration (statement –, marked with #). The figure highlights areas of strong agreement as well as topics where expert opinions varied, reflecting the complexity of certain concepts in light and health research.

Additionally, three of the original statements remained unrated by two participants, leading to missing data in the final assessment. However, even if these unrated items were categorised as ‘disagreed’, the affected statements would still not fall below the 75% consensus criterion. Therefore, they were retained in the final set of statements (table 1 and online supplemental table 1 with simplified statements and contextual information, figure 2).

Table 1. Public health messages on the non-visual effects of ocular light exposure. The statements are also available at https://lightforpublichealth.org/ (https://doi.org/10.17617/1.4a6s-ec74). Abbreviations: LED, light-emitting diode.

Number	Statement
1	Light can be described by its spectrum: how much energy there is at each wavelength across the visible spectrum (from approximately 380 to 780 nm).
2	The pattern of one’s light exposure across the day and the year can be quite complex, and depends on where one is and what one does.
3	Light exposure can be described by its intensity: The total amount of energy across all wavelengths from 380 to 780 nm, weighted according to the function of interest.
4	Daylight has what we call a broad spectrum, with a lot of energy across many wavelengths.
5	Different electric light sources (eg, LED or fluorescent lamps, etc.) have different spectra.
6	The properties of daylight (spectrum, intensity and spatial distribution) vary throughout the day and the year, and with changing weather.
7	The human eye contains the retina, which has several photosensitive cells that differ in their responses to different wavelengths.
8	The cones allow us to see colour, motion and spatial detail in bright lighting conditions.
9	The rods allow us to see rudimentary spatial detail under dim light.
10	The intrinsically photosensitive retinal ganglion cells (ipRGCs) convert light into signals that influence many physiological functions.
11	Predominantly through the ipRGCs, light causes the suppression of melatonin in the evening and at night.
12	Light is the main signal that ensures the circadian system is synchronised to the 24-hour cycles in the environment.
13	Light directly influences the biological clock in the brain, regulating sleep-wake cycles and other daily physiological rhythms.
14	Light in the morning can advance the circadian clock, and light in the evening can delay the circadian clock.
15	Light can also boost alertness and cognitive function under some conditions.
16	What determines these physiological responses to light is primarily determined by how much light reaches the retina and stimulates the ipRGCs at a certain time.
17	Higher light levels in the evening can increase the time to fall asleep.
18	Higher light levels during the daytime can improve mood.
19	Higher light levels during the daytime can improve sleep quality in the following night.
20	Following a medically prescribed protocol for bright light, exposure in the morning can lead to improvements in mood for people with certain clinical diagnoses.
21	A healthy pattern of daily light exposure includes a rhythm of bright light and darkness every day.
22	Age can influence the physiological effect of light on humans, as less light reaches the retina because of ageing.
23	There are substantial individual differences in the physiological response to light.
24	The majority of studies on the physiological effects of light have been performed in the laboratory.
25	There is a need for studies on the physiological effects of light incorporating a broad range of study populations.
26	The physiological effects of light are an area of active investigation.

The ‘Statement’ column provides detailed descriptions using technical terminology. Simplified statements and contextual information are given in online supplemental table 1.

Consensus statements for public dissemination

The consensus process resulted in the development of 26 statements that were unanimously agreed on, focusing on the effects of light on humans. These statements provide scientifically supported insights into how light influences physiological and psychological processes in addition to vision. Additionally, the consensus process involved refining these statements into simplified versions to enhance accessibility for a broader audience. Contextual information, along with relevant references, was also incorporated to provide background knowledge and real-world applications, ensuring a comprehensive understanding of the effects of light.

Evidence rating

The quality of the supporting scientific literature was assessed using a scoring system adapted from the OCEBM framework for rating individual studies. This modified approach allowed for consistent evaluation of the strength and reliability of the evidence cited supporting each statement, considering study design, methodological rigour and relevance to the topic (online supplemental table 2). Several statements were supported primarily by textbook knowledge or widely accepted biological principles, where empirical clinical trials are not feasible. This reflects a limitation in the research method rather than a lack of confidence.

Readability of statements

All scientific statements, simplified statements and contextual information were assessed for readability by an independent medical writer to ensure accessibility for a broad, non-specialist audience. The Flesch-Kincaid Grade Level for the statements was 8.6, indicating an approximate 8th-grade reading level, with a Flesch Reading Ease score of 59.3, suggesting the content is moderately easy to understand for the general public. On average, the statements contained 14.8 words per sentence and 5 characters per word. The contextual information had a Flesch-Kincaid Grade Level of 10.3 and a Reading Ease score of 43.8, reflecting a slightly more complex reading level. These sections averaged 13 words per sentence and 5.5 characters per word. In contrast, the full scientific versions of the statements were written at a Flesch-Kincaid Grade Level of 11.6, with a Reading Ease score of 42.5, indicating more advanced readability suitable for a specialist audience.

Discussion

Effective communication of scientific knowledge to the public is essential, especially when it involves behavioural changes with significant implications for health and well-being. Translating complex scientific findings about the effects of light on health into accessible information is critical for promoting awareness and informed decision-making. This public education initiative led to the creation of 26 consensus-based statements on the effects of light on humans, explicitly designed for public dissemination. A high level of agreement (>75%) was achieved for most statements among a group of scientists from different disciplines, with only two exceptions, highlighting areas requiring further discussion and clarification. The iterative process of revising the scientific statements, simplified statements and contextual information, in collaboration with the co-authors, the consortium and a medical writer skilled in communicating with lay audiences, helped refine the language to improve accessibility and understanding for diverse readers. This collaborative effort resulted in a measurable improvement in readability scores, ensuring that the final materials were not only scientifically accurate but also suitable for effective dissemination to the general public, educators and policymakers. The improvement in reading level reflects a critical step toward bridging the gap between complex scientific evidence and real-world application.

Strengths and limitations

One of the key strengths of this initiative is that it represents the first set of consensus-based statements specifically designed for public engagement on light and health. The expert-driven modified Delphi process included input from multiple disciplines, ensuring that a broad range of perspectives was incorporated. The statements underwent an iterative review and refinement process, enhancing their accuracy and clarity. Additionally, readability testing was integrated to ensure accessibility across varying levels of literacy and scientific familiarity. Collaborations with international organisations further strengthened the potential for broad dissemination and impact, increasing the likelihood that the findings will reach and influence a wide audience.

A potential limitation of this work is the presence of selection bias, as the roundtable participants represented specific disciplinary backgrounds and may not fully capture the breadth of perspectives. Participation from medical doctors was limited, which may have reduced the clinical input into the health-related recommendations. In addition, experts from low-income and middle-income countries (LMICs) were under-represented, potentially limiting the global applicability of the consensus statements. For statements 18–21, it is especially important to consider regions where high levels of ultraviolet (UV) radiation pose additional risks; in such settings, recommendations for increased light exposure must be carefully balanced with sun-protection guidance. Future initiatives could strengthen the generalisability of the statements by engaging a more diverse range of stakeholders, including greater representation from clinical experts and regional voices. Additionally, there was a necessary trade-off between scientific precision and accessibility, leading to some degree of simplification in the messaging. Further validation of the statements is needed, including public feedback and empirical testing to assess their effectiveness in real-world communication settings. Another limitation is that the process did not include a lay audience in an iterative feedback loop. Incorporating direct input from the general public can provide valuable insights into how non-experts interpret and understand the statements, improving their clarity and impact.

Assessing the quality of evidence on the effects of light on human physiology and psychology presents unique challenges. Much of the foundational work in this field comes from laboratory studies, observational research and narrative reviews, which may not meet the highest levels of evidence grading systems such as the OCEBM Levels of Evidence we used here. Moreover, the field’s interdisciplinary nature, spanning chronobiology, psychology, lighting engineering and public health, means diverse study designs and outcome measures are used, complicating harmonised assessment. While formal grading frameworks offer a structure for evaluation, they can undervalue high-quality mechanistic, observational or exploratory research. While RCTs are often regarded as the highest tier of clinical evidence, in the field of light and health, they are not always feasible, ethical or appropriate. In such cases, non-RCT designs provide indispensable insights, and lower levels in the OCEBM hierarchy do not necessarily imply weaker relevance or reliability. Additionally, several of the statements developed here represent textbook knowledge, for which a clinical trial or similar (bio)medical study designs are unsuitable. Our use of the OCEBM framework was intended to promote transparency in evidence appraisal, but the interdisciplinary nature of this field requires integrating diverse forms of evidence to inform public health messaging.

Importance of public engagement in light and health

The urgency of effectively communicating the effects of light on health stems from significant changes in the modern light environment. Increased electric lighting,²¹ prolonged screen exposure²² and a growing disconnect from natural light-dark cycles have altered human exposure patterns to light and darkness.²³

Uncertainty persists regarding several aspects of the role of light in circadian health, sleep and overall well-being. For instance, the consortium did not reach consensus on a statement addressing the extent to which daytime light exposure may counteract the effects of light at night, a topic that is only beginning to be explored in scientific research.^{24 25}

Clear, evidence-based messaging on the health impacts of light can empower individuals to make informed choices while also guiding policymakers in shaping workplace environments, urban design and lighting standards. Through well-structured communication, individuals and decision-makers can be encouraged to prioritise light exposure in ways that support circadian health and overall well-being.

Plans for implementation

Building on the consensus statements and international collaborations, the next steps involve distributing the statements through organisational networks. A key milestone was the initiative’s official launch on the UNESCO International Day of Light (16 May 2025). Further actions include (1) developing accessible outreach materials such as infographics, social media campaigns and policy briefs to support dissemination in cooperation with public-health multipliers¹⁷; (2) translating statements into multiple languages to maximise accessibility and reach diverse populations; and (3) establishing predefined indicators to monitor engagement and assess the impact of the communication efforts. Evaluation will include metrics such as website traffic, downloads and social media analytics; surveys or focus groups to assess comprehension and uptake among target audiences; and tracking references or incorporation of the statements into institutional policies, guidelines or professional recommendations. Together, these measures will enable assessment of both reach and real-world influence of the initiative. Future iterations will also involve direct testing with members of the public to optimise the clarity, accessibility and effectiveness of the communication materials.

Conclusion

This expert consensus delivers clear and accessible messages on the impact of light on human health. By translating scientific evidence into practical guidance, it serves as a valuable resource for public education and policy development. Public health stakeholders, including schools, employers, healthcare providers and urban planners, can use these messages to support healthier light exposure in everyday life. The statements emphasise the need to consider light as a crucial factor in health, alongside sleep, nutrition and physical activity.

Data availability statement

Data are available in a public, open access repository.