Circadian lighting treats light as a variable that changes over time: during the day, sufficient exposure is needed, while as night approaches it becomes important to reduce the amount of light reaching the eyes. Designing it correctly is not simply a matter of programming lamps at 2700 K in the evening and 6000 K in …
Circadian lighting treats light as a variable that changes over time: during the day, sufficient exposure is needed, while as night approaches it becomes important to reduce the amount of light reaching the eyes. Designing it correctly is not simply a matter of programming lamps at 2700 K in the evening and 6000 K in the morning. What matters is spectrum, intensity, duration, timing of exposure and the position of the light in relation to the eye. The most useful metric today for describing the melanopic component is melanopic Equivalent Daylight Illuminance, or melanopic EDI, defined by the CIE. It is a design tool, not a therapeutic promise: visual comfort, safety, glare control and light quality remain essential requirements.
What is the circadian rhythm and why does light matter?
Circadian rhythms are biological oscillations with a period close to 24 hours. In humans, a central role is played by the suprachiasmatic nucleus of the hypothalamus, which coordinates numerous physiological rhythms and is synchronised with the external day-night cycle primarily through light received by the retina.
This means that the eye is not used exclusively for vision. In addition to cones and rods, the retina contains intrinsically photosensitive retinal ganglion cells, known as ipRGCs, which contain melanopsin and contribute to light-influenced responses such as circadian synchronisation, the pupillary reflex and the modulation of alertness. ipRGCs also receive signals from visual photoreceptors, so it would be too simplistic to speak of a single “circadian receptor” separated from the rest of the visual system.
The CIE now prefers the term integrative lighting for lighting that considers both visual effects and non-visual responses influenced by light. “Circadian lighting” and “human-centric lighting” are widely used market terms, but they can refer to technically very different solutions.
Intensity, spectrum, duration and timing: the four variables to design
The relationship between light and the circadian system does not depend on a single parameter. ISO and CIE identify spectrum, intensity, duration and timing of exposure among the key variables.
Intensity determines how much light actually reaches the eye. Spectrum describes how luminous energy is distributed across different wavelengths. Duration determines how long that condition is maintained. Timing establishes when the exposure occurs in relation to a person’s biological day.
This last point is crucial. Studies on the phase response curve show that the direction of the circadian response to light varies according to timing: in general terms, light received during the latter part of the night and in the morning tends to advance circadian phase, while light in the evening and early part of the night tends to delay it. The precise effect nevertheless depends on the individual’s biological phase, not simply on the time shown on the clock.
A credible circadian lighting strategy must therefore create contrast between day and evening, rather than simply selecting a lamp that appears “natural”.
Why Kelvin values are not enough for circadian lighting design
Correlated colour temperature, or CCT, describes whether white light appears visually warmer or cooler. It is useful in lighting design, but it does not directly quantify melanopic stimulus.
Two LED sources both rated at 4000 K or 5000 K can have different spectral distributions and produce different melanopic values while delivering the same photopic illuminance. Research published in Scientific Reports has shown that, with the same CCT and illuminance, melanopic EDI can vary significantly depending on the source spectrum.
This also changes how Tunable White systems should be assessed. Automatically changing colour temperature throughout the day can be useful, but a 2700 K → 6500 K → 2700 K programme does not by itself demonstrate that the space is providing appropriate circadian exposure.
At a minimum, the Spectral Power Distribution, SPD, should be known, ideally together with the parameters defined by the CIE. Kelvin remains a description of the visual colour appearance of light; melanopic EDI instead describes a specific component of retinal stimulation.
What is melanopic EDI and why is it measured at eye level?
With the CIE S 026:2018 standard, the Commission Internationale de l’Éclairage defined a metrological system for describing stimulation of the five photoreceptor types that contribute to ipRGC-influenced responses. One of the quantities defined is α-opic Equivalent Daylight Illuminance.
In the case of melanopsin, this is referred to as melanopic EDI, expressed in lux. In intuitive terms, it indicates the illuminance produced by the CIE D65 daylight reference that would generate melanopic stimulation equivalent to that produced by the light being assessed.
Where the measurement is taken is equally important.
The CIE recommends assessing exposure in the eye region, generally on a vertical plane facing the direction of view, rather than relying only on conventional lux measurements taken on the horizontal desk plane. A room can fully comply with the required illuminance on the work surface while delivering a very different amount of light towards the occupant’s eyes.
How much circadian light is needed during the day, evening and night?
The international consensus published by Brown and other researchers in 2022 proposed a number of reference values later incorporated into the CIE Position Statement 001:2024. They refer to healthy adults who are primarily active during the day and follow regular schedules; they should not be turned into universal prescriptions for children, older people, night-shift workers or people with specific conditions.
| Time | Reference recommendation | Where |
|---|---|---|
| Daytime | ≥ 250 lux melanopic EDI | at the eye |
| Around 3 hours before habitual sleep | ≤ 10 lux melanopic EDI | at the eye |
| During sleep | ≤ 1 lux melanopic EDI | at the eye |
| Necessary activities during the night | ideally ≤ 10 lux melanopic EDI | at the eye |
These values do not replace standards relating to vision and safety. The CIE itself makes clear that integrative lighting design should complement normal requirements for illuminance, comfort and visual performance rather than override them.
Research also remains open. The CIE notes, for example, that it has not yet been established whether a given intermittent “dose” of daylight can always replace continuous exposure at the recommended level. In July 2026, it also published the new CIE TN 016:2026, specifically to make the description of lighting conditions in human studies more rigorous and comparable.
Daylight comes before Tunable White lighting
Where available, scientific consensus suggests using daylight first to achieve higher levels of exposure during the day. The CIE also recommends avoiding unnecessary restrictions on daylight entering interior spaces.
This shifts part of the design problem from the lighting system to the architecture itself.
Room depth, workstation position, window size and orientation, solar shading, partitions and reflective surfaces all affect the amount of light that actually reaches occupants. A table positioned close to a window can receive very different daytime exposure from a workstation located deep within the same room.
More light does not automatically mean better design. Glare, reflections on screens and solar overheating still need to be controlled. The CIE itself identifies glare management as one of the practical issues that must be addressed when attempting to provide high melanopic EDI values in real spaces.
Circadian lighting at home: designing for morning, daytime and evening
In the home, the first intervention is often organisational before it becomes technological.
In the morning and during the day, it is useful to spend more time in rooms with the best access to natural light: breakfast areas, kitchens, studies and home offices can make use of windows and openings without necessarily relying on complex LED systems.
When daylight is insufficient, daytime electric lighting can supplement the available contribution. Here, vertical illuminance at the eye and spectrum become important alongside conventional lux levels on the work surface.
In the evening, the design strategy changes. Rather than illuminating the whole room uniformly from above, it can be useful to work with lower light levels, localised lighting and luminaires positioned outside the main line of sight. A reading lamp must still provide adequate visibility; the aim is not to live in darkness, but to avoid unnecessarily high melanopic exposure in the hours before sleep.
Even a 2700 K light can be excessive if it is very bright and directly visible. Conversely, the right combination of intensity, geometry and spectrum can maintain good visual quality with a relatively low melanopic EDI. The 2022 consensus notes that many common warm-white domestic light sources can provide usable visual levels while keeping melanopic values low, if they are correctly specified.
In offices, the workstation should be designed from the eye’s point of view
The office is one of the environments where circadian lighting design is particularly relevant because many people spend a substantial part of the day away from outdoor light.
The first check should be how much light actually reaches the eye during the central hours of the day. Reading the average value on the desk is not enough.
Windows, workstation orientation and partitions can substantially alter exposure. Simulation studies of open-plan offices show, for example, that the geometry and height of partitions can change the circadian contribution of natural light.
Tunable White can be a useful tool, but it is not essential. Fixed-colour-temperature systems can also contribute to achieving the target when spectrum, light quantity, distribution and daylight are appropriate. Experimental studies by Lawrence Berkeley National Laboratory have shown that visual and circadian criteria can be addressed by integrating tunable LEDs and daylight while assessing illuminance, spectrum, glare and energy at the same time.
Hotels and hospitality require more flexibility than offices
In hospitality environments, the challenge increases because users do not necessarily follow the same daily schedule.
A hotel room should support different activities: evening arrival, reading, morning preparation, temporary work and sleep. A rigid “circadian” scene imposed by the building may therefore be less useful than a well-designed system combining basic automation with the possibility of user control.
During the day, daylight can be prioritised and, where necessary, supplemented by a stronger electric component. As night approaches, lower-intensity scenes and indirect or localised luminaires become more useful. During sleep, effective blackout and control of stray light sources become particularly important.
Night-time routes, bathrooms and corridors require a balance between visibility and reduced light exposure at the eye. Recommendations for low melanopic levels should never compromise safety, orientation or accessibility.
Automation and controls: what should actually change throughout the day?
An advanced control system can coordinate daylight and electric light through sensors, dimming and Tunable White sources. But the programme should modify both the quantity and spectral characteristics of the light, rather than simply moving a Kelvin slider slowly throughout the day.
A sensible sequence may provide greater exposure during active hours followed by a progressive reduction towards the evening. Daylight sensors can prevent electric lighting from remaining unnecessarily high when natural light is already sufficient; occupancy sensors can reduce energy consumption in empty spaces.
Manual override also remains important. A system designed for real people should allow adaptation to activities and visual preferences without turning automation into an imposed lighting scenario.
The most common mistakes in circadian lighting design
The first is designing only in Kelvin. CCT describes the appearance of light, but it does not uniquely determine its melanopic component.
The second is simply increasing horizontal illuminance. The circadian system responds to light reaching the retina, so measurement at the eye is central.
The third is seeking very intense “blue” light throughout the entire day. The objective is not to maximise melanopic stimulus indiscriminately, but to create an appropriate light-dark cycle. The CIE itself notes that the effects of daytime exposures far above currently recommended values require further research.
The fourth is ignoring daylight while investing in sophisticated dynamic lighting systems.
The fifth is using the same programme in homes, offices, hotels, schools and night-shift workplaces. The reference recommendations primarily concern healthy adults with regular daytime activity; shift workers, children and older people require different considerations.
The sixth is attributing guaranteed medical benefits to the design. CIE S 026 is a measurement system, and the organisation itself makes clear that the standard does not provide a complete quantitative prediction of individual responses, nor does it replace clinical or safety guidance.
What to ask for when specifying a circadian lighting system
A commercial label such as “Human Centric”, “biodynamic” or “circadian ready” is not enough.
To assess a solution seriously, it is worth requesting at least:
- SPD of the light sources in the different operating configurations;
- melanopic EDI or melanopic DER values according to CIE S 026, not CCT alone;
- expected vertical illuminance at eye level and in the direction of view;
- photopic illuminance levels required for the relevant tasks;
- colour quality of the light and its behaviour across different settings;
- luminance distribution and glare control;
- dimming behaviour and temporal light modulation;
- control logic, schedules, daylight integration and manual adjustment options;
- final on-site verification, because values calculated at luminaire level do not necessarily match those actually received by the occupant.
The most interesting aspect of circadian lighting in 2026 is precisely this shift from scenography to measurement. A lamp that becomes cooler at nine in the morning and warmer at nine in the evening may create a pleasant atmosphere, but that alone does not prove that the project follows the natural rhythm of the day.
A more rigorous design approach starts instead with architecture and daylight, measures what actually reaches the eyes, considers spectrum, intensity, timing and duration, and uses electric lighting to supplement what natural light cannot provide. It is a less spectacular approach than some narratives around “wellness lighting”, but one that is far more closely aligned with the current state of research and CIE guidance.