ReviewNon-visual effects of indoor light environment on humans: A review✰
Introduction
As an important characteristic of indoor environment, light affects human health and well-being [1]. Increasingly more human activities are confined to indoor environments, which are vulnerable to the influence of various light sources, including light-emitting diodes (LEDs) as a part of new generation lighting products. As a result, increasing attention has been paid to creating a comfortable and healthy lighting environment in offices, classrooms, or homes due to intense work and social interactions [2, 3]. Utilising modern lighting control technology to sufficiently provide humans with psychological and physiological comfort is an important means to improve their work efficiency and well-being [4], [5], [6].
Light is one of the significant factors in people's daily lives and is involved in most human activities. When visible light (electromagnetic spectrum between 380 nm and 780 nm) enters the eye and reaches the retina, a complex chemical reaction occurs in the photoreceptor cells; rods and cones. The reaction is termed phototransduction. Light activation causes a graded change in membrane potential and a corresponding change in the rate of transmitter release onto postsynaptic neurones [7]. When visual information leaves the retina, it is sent via the optic nerve to the dorsal lateral geniculate nucleus in the thalamus, and then transmitted to the primary visual cortex, which is located in and around the calcarine fissure in the occipital lobe, thus causing vision [7]. However, photoreception in the eye leads not only to vision but also to effects on human physiology, behaviour, and mood, often summarised as non-visual effects of light [8]. It is also known as non-image forming effects in the literature. In 2001, Thapan et al. [9] and Brainard et al. [10] demonstrated evidence of non-rod non-cone photoreceptors. Since the discovery of intrinsically photosensitive retinal ganglion cells (ipRGCs) by Professor Berson from Brown University in 2002 [11, 12], the non-visual effects of light on humans have aroused much attention. These ipRGCs were first recognised in relation to their role in regulating circadian rhythms, and ipRGCs also influence many other processes [8]. Researchers have created different lighting conditions in experiments to evaluate the magnitude of the non-visual responses through various biomarkers of the circadian system or indicators of human performance, such as melatonin suppression, phase shift and sleep quality, subjective and objective alertness, and mood [13]. These experimental studies serve as the basis for a better understanding of the non-visual effects and provide guidance for lighting designs and the operation of lighting systems that consider human health and well-being.
The light factors influencing non-visual effects have been discovered in laboratory experiments. After referring to two reviews [14, 15] and a modelling framework [16, 17], P. Khademagha et al. [13] summarised six factors triggering the non-visual light effects, including spectrum (spectral power distribution, SPD), quantity, directionality, timing, duration, and history, which are grouped into luminous and temporal categories. In 2016, the International Commission on Illumination (CIE) published a technical report on healthful interior lighting recommendations that delivered a research roadmap of questions for technical researchers [18]. Understanding the relationship between light exposure and its effects on the circadian rhythm, alertness, and feelings of well-being is the precondition for creating a healthy and comfortable light environment. This topic encompasses an interdisciplinary study of life science, ergonomics, and behavioural and cognitive neuroscience, and is aligned with traditional lighting. In recent years, the catchphrase “Human-Centric Lighting” (HCL) has come to describe lighting that is intended to address non-visual effects. With the birth of the concept of HCL, every user of a lighting system is considered individually in accordance with their age, profession, and current activity as well as external parameters, such as weather, time of day, and the presence of daylight [19]. Precisely, while considering the lighting quality and lighting biosafety, the individual needs for lighting should be taken into account in different situations at different times. Researchers generally believe that systematisation and a long-term strategy of ambient lighting are two key points and that human-centric lighting design and operation control will improve the productivity and health of individuals in the next generation of indoor light environments [19], [20], [21].
There are already several reviews focussing on lighting and health, which include non-visual effects. For example, Souman et al. [22] performed a systematic review of empirical studies between 1990 and 2016 on the acute alerting effects of light, and summarised important conclusions on the evaluation of effects when light intensity and spectral distribution were manipulated. Figueiro et al. [23] described recent applied and field research and summarised the lighting characteristics that affect the outputs of the circadian system in 2018, which helped lighting researchers and professionals to determine how it can be employed to maintain circadian entrainment. Khademagha et al. [13] presented a theoretical framework of the relationship between non-visual effects and light from the perspective of relevance in 2016, which enabled daylighting stakeholders to incorporate the non-visual light requirements into their design. However, there is no comprehensive analysis of the impact of specific light factors on three aspects of non-visual effects: circadian rhythm (melatonin suppression, phase shift, and sleep quality), subjective and objective alertness, and mood. The authors believe that this work will help to improve the light environment to meet the requirements of human health and well-being, and provide a reference for the application of health lighting technologies considering non-visual effects simultaneously.
In this work, we provide a review of the non-visual effects of indoor light environment on humans based on the existing literature. The impact of light intensity, spectral distribution, time of day, and duration on three aspects of non-visual effects, including circadian rhythm, alertness, and mood, are summarised and analysed according to scientific selection and induction. The findings have an important reference value for the optimisation of lighting in the built environment for both lighting design and lighting control. It can also prompt researchers in these fields of life science, ergonomics, and behavioural and cognitive neuroscience to further promote interdisciplinary integration for the demanding targets of building science, and provide more detailed experimental data and theoretical bases for the design and operation of the light environment.
Section snippets
Search procedure
The scope of the review was the previously reported research outcomes. The goal of this literature review was to summarise and integrate findings. A conceptual organisation was used to organise the review, and scholars and stakeholders were intended to be the audience.
The search focussed on two main concepts: non-visual effects and light. Therefore, the terms for searching included combinations of “non-visual effects” and “light*” or “non-image forming effects” and “light*”. The terms were
Aspects of non-visual effects
In the experiments of the selected publications, the non-visual effects of light on humans were evaluated based on different aspects. In this review, circadian rhythm, alertness, and mood were investigated because these three aspects were the most assessed aspects among the studies (63%, 63%, and 48%, respectively). The specific instructions are shown in Fig. 2.
In the circadian rhythm, melatonin suppression has been widely researched. When light hits the retina, the ipRGCs are activated by the
Results
In this review, four light factors are involved according to the literature selected in Section 2.2 Study selection. Two of the factors are included in luminous factors: quantity and spectrum (SPD). Different light manipulations were performed in the selected studies. For polychromatic white light, illuminance and CCT were changed; for monochromatic light, the intensity and wavelength were changed. The other two temporal factors are timing and duration. As indicated in the introduction,
Discussion
The quality of eligible studies was assessed through an assessment tool, and the criteria included study question, eligibility criteria and study population, outcome measures clearly described, valid and reliable, statistical analysis, etc. Although there are different technical differences in the same research objectives, the distribution rules of the conclusions are reasonable. However, owing to the problems of experimental conditions and methods, the research conclusions under certain
Conclusion
In this review, we focussed on the outcomes of light factors on three aspects of non-visual effects. Through 27 publications with strong relevance, circadian rhythm, alertness, and mood were investigated because these three aspects were the most assessed aspects among the studies (63%, 63%, and 48%, respectively). Each aspect based on four different light factors, especially quantity and spectrum, was scientifically summarised and analysed. The key findings are as follows:
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The increase in
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
Acknowledgements
This work was supported by the Fundamental Research Funds for the Central Universities under Grants 22120180189 and 22120170260, China.
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✰Declarations of interest: none