Light exposure has important effects on human health and wellbeing, regulating our sleep/wake cycles, and influencing our mental and physical health. Advances in our understanding of the role of light in health, and emerging lighting technologies, now present opportunities to adjust lighting to promote optimal physical and mental health and performance. In this blog, Professor Rob Lucas and Professor Tim Brown explore how we measure the influence of light on physiology, sleep, and alertness in healthy adults, and what this means for policymakers and regulators when setting minimum and maximum levels of healthy light.

• Poor sleep is estimated to cost the UK the equivalent of 2.1% of GDP each year by 2025, through lost productivity, absenteeism, and accidents.
• Exposure to the right kind of light will reinforce good circadian rhythm alignment (an internal process that regulates the sleep–wake cycle) and may help promote restful sleep, cognitive performance, and long-term health.
• We have developed a new international standard of light exposure for policymakers and regulators to use when setting targets for healthy levels of light.

Industrialisation and urbanisation have progressively altered individuals’ exposures to light, resulting in less natural light during the daytime – and less darkness during the night – as a result of spending more time indoors where electric lighting is the dominant source of illumination.

The light we receive through our eyes has a huge influence on our health and wellbeing, regulating our sleep/wake cycles, hormone levels, and chemical activities in our brain. Many individuals don’t receive a healthy exposure to the right kind of light at the right time, with negative repercussions on sleep, productivity, and health, ranging from increased accident risk to a greater chance of developing diabetes, heart disease or some forms of cancer. As such, there is an urgent need for evidence-led recommendations to inform the design and application of healthy light exposure.

What kind of light do we really get?

It has long been recognised that our existing approaches for measuring light, which reflect perceived brightness for humans (e.g. lux) are not suitable for capturing the ability of light to influence physiology, sleep and health. Working with international colleagues, we have developed a global standard for measuring the influence of light on the receptors in our eyes that detect it. Doing so means we can now provide scientific consensus recommendations for lighting in an easily-measured quantity – melanopic equivalent daylight illuminance (mEDI). This means the light that interacts with light-sensitive cells in our eyes that control our biological timing – it doesn’t include all light (as humans cannot perceive the ultraviolet or infrared spectrums) and places greater weight on shorter (‘bluer’) parts of the spectrum than for conventional measures of perceived brightness.

There are several national and international standards that are relevant to indoor light exposure in a given environment. Existing guidelines for lighting installations in indoor places primarily focus on visual function, including visual comfort, visual performance, and seeing safely for people with normal, or corrected-to-normal vision. While the electrical light sources in most common use are optimised for their visual qualities, the light they provide leaves us with an indoor light environment that is potentially suboptimal for supporting human health, performance, and well-being since, together with the focus on energy saving, they restrict illumination indoors to levels that are typically at least 10-fold lower than the natural light environment outdoors.

What kind of light should we be getting?

In the daytime, the recommended minimum mEDI is 250 lux (a unit of illumination; an overcast day is around 1,000 lux), measured at eye level when seated. If available, daylight should be used in the first instance to meet these levels. Increasing light exposure during the day in line with our recommendations has been shown to benefit alertness, performance, and sleep in a wide range of real-world settings, even in the presence of daylight or stimulants such as caffeine.

In the evening, starting at least 3 hours before bedtime, the recommended maximum mEDI is 10 lux, while at bedtime, the sleep environment should be as dark as possible – a maximum mEDI of 1 lux. Increased exposure to electric light in the evening and night is considered to cause adverse effects on circadian rhythms and health outcomes. Indeed, even relatively low levels of light in the sleep environment (conservatively, mEDI >3 lux) have been associated with impaired sleep and increased incidence of diabetes in large cohort studies.

Asides from the possible impacts on health associated with regular sleep-wake cycle disruption (such as cardiovascular disease, diabetes, and cancer), benefits associated with improved sleep alone could be substantial. Insufficient sleep is estimated to cost the United States 2.4% GDP, due absenteeism, accidents, and reduced productivity, while in the UK, this figure is expected to be around 2.1% of GDP by 2025. Even relatively modest improvements for those with poor sleep (<6 hours sleep a night increased to 6 to 7 hours) are predicted to increase US GDP in the order of 1.7%, or approximately 300 billion USD a year.

How can policymakers act?

Meeting the recommendations is expected to benefit health and well-being for any individuals whose current daytime light exposure is below, or evening/night-time exposure above, the specified targets (collectively a sizeable majority of the population).

An important consideration in achieving our recommendations for daytime settings is whether this would necessitate higher overall light levels, and therefore increase energy expenditure and/or the risk of visual discomfort. Importantly, however, in most cases the recommendations can be met by adjusting the composition (rather than overall amount) of available light. This can be achieved through approaches such as increasing the availability of natural daylight, optimising lighting design to provide a more appropriate wavelength composition while maximising energy efficiency and minimising visual discomfort, and adjusting finishes and furnishings to optimise surface reflectance.

Adherence to our evening/night-time recommendations should further promote decreases in energy usage by helping to reduce the use of unnecessarily high levels of illumination outside of the day. This should be should be widely achievable without compromising important considerations around visual comfort or safety by appropriate lighting choices, such as employing warmer white lighting in the evening, use of blackout blinds in the bedroom and appropriate task lighting where required.

Collectively, the recommendations are intended to inform lighting design considerations for typical, real-world environments such as offices, schools and colleges, care homes, and in- and out-patient settings. In the UK, the Workplace Regulations, which cover health, safety, and welfare, currently only mandate that lighting should “be sufficient to enable people to work and move about safely”. As such, there is scope for policymakers and regulators – such as the Health and Safety Executive and the Department for Work and Pensions – to introduce new guidance on healthy mEDI levels.

In hospitals and care homes, the Care Quality Commission and the NHS can carry out a similar role to ensure the light environment is conducive to health and recovery, while the Ministry of Justice and HM Inspectorate of Prisons may do the same for prisons and police custody cells, with additional considerations around safety and security.

As noted above, application of our recommendations across such settings is facilitated by the free availability of tools for calculating melanopic EDI (and also estimating this given known illuminance and type of lighting), and the emergence of low-cost commercial sensors for direct measurement of mEDI, similar to “lux meters”.

Policymakers of course cannot regulate how people light their own homes – but a public information campaign could help people to make better choices in lighting. And where the Government can act – healthcare settings, schools, prisons, and other public spaces or areas subject to the HSE – they should move to implement these recommendations, with direct benefits for the nation’s health.