The Ultimate Guide to Circadian Lighting

Jonathan Rush, director of lighting design with the Tetra Tech High Performance Buildings Group, examines and explores research into the effects of light exposure to the human body and the potential benefits and consequences of incorporating this research into lighting design.

The impact of lighting design on the interior environment has never been more important.

Solutions have moved from the functional to the aspirational, with a growing trend for spaces to be designed to facilitate occupants’ enhanced health and well-being.

A vital aspect of health and well-being is healthy sleep. Disrupted, poor, or inconstant sleep can wreak havoc, increasing the risk of serious illnesses, impacting our alertness, mood, and mental wellness.

Sleep and Circadian Rhythms

The human body has 37 trillion cells, each with its own time clock. For people to sleep properly, it’s vital that each cell is properly synchronized. This is the function of something called the suprachiasmatic nucleus (SCN), a tiny area of the brain located within the hypothalamus.

Human circadian rhythm regulates a 24-hour cycle of sleep, wake, hunger, alertness, hormone release, and body temperature… and generally keeps each person healthy.

But this rhythm can become desynchronized and have major impacts on health and well-being, leading to higher rates of obesity, diabetes, heart attacks, and cancer.

Light's Influence

Light is one of the biggest influences on circadian rhythm:


A chart that explains the way eyes process light and send messages to the brain to signal sleep.


How light helps us sleep
Photosensitive Retinal Ganglion Cells: One. The retinas have non-image forming cells called photosensitive retinal ganglion cells. These cells have one job: to receive light and subsequently determine if it is night or day.
SCN: Two. These cells are more sensitive to blue light and, should they not receive adequate light for two hours, send a message to the SCN.
Pineal Gland: Three. Next, the SCN communicates with the pineal gland, which starts the secretion of the melatonin hormone.
Melatonin: Four. The increase in melatonin induces sleep, triggering the cycles of hormone release necessary for healthy sleep.

Ultimately, lighting—both daylight and artificial—plays a vital role in people’s circadian rhythms, affecting human health and well-being.

The potential well-being impacts that this relatively recent understanding brings to the lighting design of interior spaces is exciting. Considering the time we collectively spend in lit buildings, it is important that the possibilities for circadian-centric lighting design within occupied spaces are considered in the design process.


An arc-shaped diagram showing color spectrum changes in light throughout the day.


Natural light: An arc-shaped diagram with a color gradient, starting from the left end with the color black (night), shifting into orange (morning), then to blue in the middle (midday), shifting back to orange (afternoon), and then fading back to black (night) again on the right end.

The simplified diagram above shows how daylight changes from a warmer color spectrum in the morning, to blue—the dominant spectrum in daylight—during the day, and then back again in the evening.

The human eyes and brain receive prompts from this natural day/night cycle and tell the body when to be awake and when to sleep. Unsurprisingly, bright blue light in the evening can disrupt sleep, and dark, warm-light interiors during the day can reduce alertness.

Intense bursts of blue (or even red) spectrum light can temporarily increase alertness and concentration in a similar way to a cup of coffee, acting as a stimulant.

In circadian lighting terms, the most sensitive wavelength appears to be the blue spectrum: about 480 nanometers (nm). However, the power of blue spectrum lighting has been known for quite some time; in the late 1990s, research by lighting manufacturers suggested a jolt of blue spectrum light as a way to stimulate workers.

The research not only highlights the potential potency of blue light, but also raises ethical questions.

A Trio of Drivers

With the demand for well-being-focused buildings and spaces, the circadian system and its impacts on human health have been researched in greater depth.

LED light sources have the ability to dim and color mix, enabling production of new dynamic lighting systems that can mimic the spectrum of daylight—circadian lighting.


One of three identical images of an AstraZeneca office space in varying shades of lighting.


Our circadian lighting design for AstraZeneca, in spaces where natural daylight could not be the dominant light source. Instead, a circadian control system cycles through from warm to cool white light.

A circadian lighting system is, in essence, a system that takes its cues from natural daylight. Daylight changes throughout the day and in response to climate and geography. However, during general waking hours, it is high in blue spectrum and bright—over 10,000 lux and up to 100,000 lux on a sunny day. As the sun sets, the blue spectrum falls, and the light becomes warmer and less bright. This is a great indication that people need more blue spectrum light while awake and warmer, dimmer light closer to sleep.

The natural environment also suggests there are benefits from darkness or near darkness for sleep.

But for an artificial lighting system that imitates the characteristics of natural light to be usable, these clues needed to be pieced together into a definable solution—one that could be specified and calculated in a similar way to a normal lux calculation.


Three interior buildings showing different qualities of lighting, including daylighting from skylights and large windows.


Quantifying the exact spectrum required for melatonin suppression has begun to be answered by scientists at the University of Manchester.

Illuminance Sensitivity Curve

The University of Manchester first proposed the melanopic illuminance curve of eye sensitivity in 2011.

In 2014 this curve was expanded by the same team to include all of the contributing cones and rods in the eye. Within the retina of the eye, rods take in low light, while cones are responsible for us seeing in color. Each one has its own sensitivity to light spectrum and each forming part of our visual and non-visual responses to light. This is called the α-opic illuminance sensitivity curve.


A graph showing the different illuminance sensitivities of the cones and rods of the human eye.


The α-opic illuminance sensitivity curve, showing the different illuminance sensitivities of the cones and rods of the human eye.

This curve represents a solid model for building the light spectrum appropriate for an artificial luminaire. It was the closest the lighting design industry had to quantifying the exact spectrum required for melatonin suppression—the Equivalent Melanopic Lux (EML).

EML formed the basis of the recommendations in the German Institute for Standardization’s standard DIN Spec 6760, which also offers metrics for biologically effective lighting.

Later on in 2020, the EML was replaced and with the Melanopic Equivalent Daylight Illuminance (m-EDI). EML did not provide an SI unit for calculating lux, so it was modified to an equivalent SI model of illuminance.

Since the output melanopic ratio of m-EDI is a little more onerous than EML, being about 10 percent less than m-EDI, this suggests some of the lower previous recommendations of the WELL Standard may need to be reconsidered. However, in general, the m-EDI metric represents a valuable step in the journey.

In late 2019, a collection of the world’s leading neuroscientists, psychologists, and chronobiologists gathered in Manchester for the second International Workshop on Circadian and Neurophysiological Photometry. The outcome was a series of m-EDI values, which suggested a minimum value required throughout the day and maximum values in the evening and while sleeping.

These minimum and maximum values offer great opportunities in all aspects of the built environment and give further information on how much light the human body needs. Like all illuminance guidance, they are not definitive as everyone is different, but they do offer a framework around how healthy exposure to light throughout the day can be evaluated.


An eye diagram demonstrating the recommended amounts of exposure to light during daytime, evening, and night.


Light Recommendations
Daytime: Indoor environment. Minimum 250 lux m-EDI vertically at the eye. Light enriched in blue spectrum like daylight.
Evening: Residential and other indoor environment. Maximum 10 lux m-EDI vertically at the eye 3 hours before bedtime. Light warm in spectrum like a candle with no blue spectrum.
Night: Sleep environment. Maximum 1 lux m-EDI at the eye as we sleep. Ideally no light, but if light is required, it should have no blue spectrum to maximum 10 lux m-EDI.

The Ethics of Circadian Lighting


Three images: basketball hoop with yellow lighting, lantern with red lighting, person’s silhouette with blue lighting.


The data requires careful thought before designing circadian lighting systems.

A striking quote from the Lucas et al. 2014 paper should act as a guiding compass for lighting design and all related endeavors:

“In many ways, light can be considered a drug, having the potential for both beneficial and deleterious effects. These conflicting effects can occur concurrently, and in a single individual and context.”

If circadian lighting can be used to manipulate a brain on a biological level, then there must be caution with how it is used.

Arguably, the ethical line falls between whether the color spectrum of light is being used to augment or support users:

Augment: Using light unnaturally to stimulate a physiological response to achieve greater productivity or concentration.

Example: very cool spectrum or blue light during the day to saturate the eye with 480 nm light and overstimulate alertness.

Support: Using reasonable, considered lighting design to support a natural and considered light color exposure.

Example: lights that go warmer in color and dim into the evening to support natural tiredness and sleep.

Personal Differences: One major issue is the one-size-fits-all approach.

People are different—from sleep schedules to geography, work patterns, diet, age, and subjective responses to light. We know the eye yellows and receives less blue spectrum light with age, and that a person of 50-60 years secretes about 35 percent of the melatonin of a 10-year-old.

We also know that personal preference and light quality is important in a person’s appreciation of their indoor environment. While the science may favor cooler spectrum lighting during the day, individual preference for lighting, and thus perception of illuminated quality, may contradict this.


Another key variable is illuminance. Modern building standards have consistently tried to lower illuminance levels, as a means of reducing building energy usage, and allow more variation and creativity in illuminated interior environments. But research suggests that higher illuminances are more supportive to melanopsin.

This is a major conflict for building energy loads seeking to reduce energy consumption and, hence, reduce carbon footprint.


Research suggests that natural light or daylight can have a positive impact on well-being, mood, and the human eye, and no artificial circadian solution can compete with natural light for its benefits. Daylight may have a regenerative quality for the eyes, and certainly time indoors and away from natural light has been seen to increase myopia rates in children. Any circadian lighting solution that reduced exposure to daylight or reduced reliance on quality daylight in architecture would be detrimental for general and ocular health.

In addition, color mixing between color sources requires dimming, and dimming can cause flickering. Many electronic drivers for LEDs have high flicker modulation, similar to that of old fluorescent lighting that anecdotally has caused headaches. A dimming or color-mixing system has the potential to cause detrimental flicker in the lit space.

A Human-centric Future for Lighting

While there are still many unanswered questions regarding circadian lighting and its adoption, the potential is truly exciting.

Quality lighting solutions are vital for creating spaces that work for people. For many years, the psychological, and sometimes physiological, human responses to light have been intuitively understood, but this recent scientific understanding takes the potential uses and impacts of lighting to new levels.

The industry also has seen research-backed evidence that lighting designed to meet human physiological and biological needs can support the circadian system, enhancing alertness and productivity throughout the day and encouraging quality sleep into the evening and night. This research has shown that light can benefit the health and well-being of office workers, school children, hospital patients and clinical staff, elderly people, and night shift workers.

In fact, there are benefits in all areas of human interactions with light, and there is a greater understanding of the needs of people via either daylight or artificial light throughout our 24-hour day.

And we are here to assist.

To realize the benefits afforded by these exciting new technologies, lighting designers and engineers must also consider the key challenges of circadian lighting, taking care to weigh the unknowns and concerns, before producing designs and installations that claim to support a healthy circadian rhythm.

In seeking to influence the physiology of people through light, the possibility also exists of losing sight of the primary objective of lighting: to create attractive and comfortable spaces that benefit people and improve the built environment.

Tetra Tech lighting design experts apply the latest research, standards, and experience in this exciting and developing area of research to building designs globally and provide successful built outcomes.

As a lighting designer, our focus is on well-being by Leading with Science®—understanding light’s impact and how it can enhance a space aesthetically, emotionally, and physiologically.


A diamond graphic explaining the components of lighting design: aesthetic, emotional, physiological, and ecological.


Human-centered Lighting
Aesthetic: creating spaces we want to use and be in.
Emotional: understanding the intrinsic, personal, and emotive relationships we have with light.
Physiological: the impact of the light on the functions of our body. And the importance of our perception on these functions.
Ecological: the impact of the light on our society, environment, and biodiversity.
By using this quartet of considerations, we can craft a lighting environment that is truly human-centered.

Jonathan rush

Jonathan Rush

Jonathan is an architectural lighting designer with more than 20 years of experience. He believes that human beings have a physiological and emotional response to light that must be understood to create engaging experiences. He is passionate about our ecology and creating a kinder society. For him, lighting is an amazing medium for art and design and one that can provide the most incredible experiences in architecture—putting people and ecology at the heart of design. He is a fellow of the Society of Light and Lighting and an active participant in the lighting design industry.