How to Measure the Effectiveness of Circadian Lighting?
Since the discovery of photosensitive retinal ganglion cells influence the body’s circadian rhythms, there has been a desire to establish a reliable metric that reveals the true effect of circadian lighting. To read more about circadian rhythms and retinal ganglion cells, click here.
Over the years, much scientific research and studies have revealed qualitative evidence to show how lighting has improved building occupants experience; however, as an industry, we lack the quantitative data - solid metrics that clearly show how the effects of circadian lighting can be measured; this would also be beneficial for establishing a quantifiable return on investment.
We will discuss some of the various models that have been developed below:
The WELL Building Standard
WELL’s measure of features within the built environment that impact human health and wellbeing has led to the development of a standard that architects and designers can follow - it is important for designers to consider the effects of lighting on occupant health and wellbeing and not just focus on the aesthetic of the lighting, it must be suitable for the tasks that will be carried out in the space.
WELL focuses solely on the health and wellbeing of building occupants. It covers a range of areas, including air, water, nourishment, light, fitness, comfort and mind - all with the aim of improving nutrition, fitness, mood, sleep, comfort and performance.
The ‘light’ segment of the standard aims to minimise disruption to human circadian cycles; it sets out guidance that supports a good sleep pattern, improves productivity within the workplace, and provides a higher quality of illumination for the various tasks completed in different workplaces. The diagram below summaries features which should be complied with:
The dark blue circles are mandatory requirements. The lighter blue circles can be added to enhance the occupant experience further, amounting to more credits contributing to a buildings WELL score.
But how does WELL work? What did they actually measure in order to establish this guidance…ultimately, how do you measure the effect of circadian lighting?
Feature 54 in the diagram is all about creating environments that minimise disruption to human circadian cycles. It uses a metric called Equivalent Melanopic Lux (EML), which quantifies light effects on human circadian cycles. It ‘can be calculated at a point and in a given direction by multiplying the visual illuminance (in lux) by the melanopic ratio, which depends on the spectrum of incident light.’ ‘The melanopic ratio is the ratio of the melanopic response (of the cells in the eye that help control the circadian rhythms) to the visual response’. (Lux Review, 2018).
Circadian Stimulus
‘In 2005, Rea et al. proposed a mathematical model of human circadian phototransduction…By using this model, the density of light measured at the cornea is converted into circadian light (CLA), which is comparable to conventional photopic illuminance but weighted by the spectral sensitivity of the human circadian system as measured by acute melatonin suppression after a 1-hour exposure. The CLA value can then be used to determine circadian stimulus (CS), which reflects the effectiveness of the spectrally weighted density of light incident at the eye from no suppression, or threshold (CS = 0.1), to saturation (CS = 0.7)’ (Figueiro 2017).
‘There are five important characteristics of light for both the human visual and circadian systems: quantity, spectrum, timing, duration, and distribution’ (Figueiro 2017). But the way light is used to support human circadian cycles compared to how light is used for just vision purposes is different.
Research with Alzheimer’s disease patients, office workers, teenagers and healthy older adults revealed that exposure to a CS of 0.3 or greater at the eye, for at least one hour in the early part of the day, is effective for stimulating the circadian system and is associated with better sleep and improved behaviour and mood (Figueiro 2016).
A CS Calculator has been developed to help lighting professionals design spaces that optimise the health and wellbeing of occupants. The ‘tool is offered to facilitate calculations of CLA and CS for several example light source spectra as well as user-supplied light source spectra’ (Figueiro 2016).
Circadian Action Factor
Gall and Bieske developed the Circadian Action Factor (CAF) as an indicator of the non-visual effects light has on human health and wellbeing.
The Circadian Action Factor (CAF), simply, ‘refers to the biological action per unit of visual response. Different CAF values are advisable depending on the time of day: high CAF during daytime and low CAF during nighttime. CAF values can be used as an optimizing tool for artificial light for human health’ (Hye et. al. 2014)
‘Gall proposes ideal combinations between individual activities, the Circadian activation factor acv and colour temperature of light sources. This is a coefficient that roughly describes the circadian efficiency of the various colours of light, yet it doesn't consider the phenomenon of spectral opposition’ (Busatto et. al. 2020).
WELL Building standard is about supporting broad Human-centred design (including lighting and the balance with natural light wherever possible). The Circadian Stimulus and Circadian Action Factor metrics are much more about quantifying the proactive effect of differing light on the Circadian cycle.
It is also worth noting the difference between measuring Lux and Melanopic Lux. The definition of ‘Lux’ is ‘the SI unit of illuminance, equal to one lumen per square metre’ (Lexico n/a), basically the measure of how well an area is illuminated. Whereas Melanopic Lux is described as a way of measuring the biological effects of light on humans. It is an alternate metric weighted to the photosensitive retinal ganglion cells instead of the cones, which is the case with traditional lux. During Performance Verification, Melanopic Lux is measured on the vertical plane at eye level of the occupant (WELL 2020).
Many other models have been developed over the years, and each one model attempts to provide a convenient, usable assessment method. However, it has been suggested that no one model has yet emerged as ideal. However, there are clear ways to measure the effect of circadian lighting on building occupants and designers and architects have begun to use these models to help them ensure that environments are circadian supportive - the demand for this is expected to continue to grow, especially as we move forward in a post COVID world. Therefore, there is definitely an opportunity to develop a model that is adopted industry-wide and is a consistent measure across all buildings. This would be a crucial advancement for the future of the circadian lighting industry.
Ready to install circadian lighting in your office or other building where the circadian rhythm can have a huge benefit of the occupants wellbeing? Get in touch here.
Bibliography
LOWRY G 2018. A comparison of metrics proposed for circadian lighting and the criterion adopted in the WELL Building Standard. School of the Built Environment and Architecture https://openresearch.lsbu.ac.uk/download/40bab6a72fef47a72ee17721b2f1f407fbc6ac3eafbcae6eb2588f297b02dc39/361753/032%20final.pdf. Accessed 02/07/20.
LUX Review, 2018. How to design lighting to the WELL standard. https://www.luxreview.com/2018/01/18/how-to-design-lighting-to-the-well-standard/. Accessed 02/07/20.
Figueiro, G, M et al., 2016. Designing with Circadian Stimulus. https://www.lrc.rpi.edu/resources/newsroom/LDA_CircadianStimulus_Oct2016.pdf. Accessed 02/07/20.
Figueiro, G, M 2017. Disruption of Circadian Rhythms by Light During Day and Night. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5568574/. Accessed 01/07/20.
zheng, Li-Li & Wu, Ting-Zhu & Lu, Yi-Jun & Gao, Yu-Lin & Wang, Ya-Jun & Zhu, Li-Hong & Guo, Ziquan & Chen, Zhong. (, 2016). Spectral Optimization of Three-Primary LEDs by Considering the Circadian Action Factor. IEEE Photonics Journal. PP. 1-1. 10.1109/JPHOT.2016.2623667.
Hye Oh, J., Ji Yang, S. & Rag Do, Y. Healthy, natural, efficient and tunable lighting: four-package white LEDs for optimizing the circadian effect, colour quality and vision performance. Light Sci Appl 3, e141 (2014). https://doi.org/10.1038/lsa.2014.22
Busatto Nicola, Mora Dalla Tiziano, Peron Fabio, Romagnoni Piercarlo, Application of Different Circadian Lighting Metrics in a Health Residence, Journal of Daylighting 7 (2020) 13-24. http://dx.doi.org/10.15627/jd.2020.2
Lexico N/A. Lux. https://www.lexico.com/definition/lux. Accessed 06/07/20.
International WELL Building Institute 2020. CIRCADIAN LIGHTING DESIGN. https://standard.wellcertified.com/light/circadian-lighting-design#:~:text=The%20biological%20effects%20of%20light,eye%20level%20of%20the%20occupant. Accessed 06/07/20.