Circadian lighting is a lighting approach that considers how light exposure changes throughout the day — not only for vision, but also in relation to the human circadian system.
Effective circadian lighting design goes beyond shifting from cool white in the morning to warm white in the evening. It considers spectrum, intensity, timing, duration, spatial distribution, daylight, and the amount of light that actually reaches the eye.
For architects and lighting designers, this changes the way a lighting system is evaluated. Horizontal illuminance, CCT, CRI, and visual appearance remain important, but they do not fully describe how a person is exposed to light throughout the day.
This guide explains the science behind circadian lighting, key metrics such as melanopic EDI, EML, and Circadian Stimulus, relevant design frameworks including WELL, and practical considerations for specifying a circadian lighting system.
For a broader discussion of the role of lighting in wellbeing-focused architecture, see our Professional Guide to Circadian Lighting & Human-Centric Lighting.
Why Circadian Lighting Matters
Human physiology developed under a strong natural light-dark cycle: relatively bright light during the day and much lower light exposure after sunset.
Modern indoor environments can create a very different pattern. People may spend much of the daytime under relatively low indoor illumination while continuing to receive electric light late into the evening.
Light reaching the eye can influence circadian timing, melatonin regulation, sleep and wake patterns, and acute alerting responses. The effect depends not on one lighting parameter, but on the overall exposure: when light is received, for how long, at what intensity, and with what spectral composition.
This is why circadian and Human-Centric Lighting strategies increasingly appear in workplaces, healthcare, senior living, hospitality, residential environments, and building frameworks such as WELL.
The objective is not to create a supposedly “healthy” color temperature. It is to establish an appropriate daily pattern of light exposure.
How Light Influences the Circadian System
Human vision is primarily associated with rods and cones, but the retina also contains intrinsically photosensitive retinal ganglion cells (ipRGCs) that contain the photopigment melanopsin.
These cells play an important role in non-visual responses to light, including circadian entrainment and pupil regulation. They also receive input from rods and cones, meaning that human responses to light are the result of an interconnected retinal system rather than a single independent photoreceptor.
Melanopsin is particularly sensitive to light in the blue-cyan region of the visible spectrum, with peak sensitivity close to 490 nm. This spectral response forms the basis of the melanopic quantities defined in CIE S 026.
For lighting design, four principles are especially important.
Spectrum Matters
Two light sources with the same CCT can have different spectral power distributions and therefore different melanopic characteristics.
A 4000K source, for example, is not biologically defined simply because it is labeled 4000K. CCT describes the visual appearance of white light, not its complete spectral composition.
Intensity Matters
The biological relevance of a spectrum depends on how much of that light actually reaches the eye.
A source with a relatively high melanopic ratio used at very low illuminance may produce less melanopic exposure than another spectrum delivered at substantially higher intensity.
Timing and Duration Matter
Light exposure is interpreted in the context of time.
Relatively high daytime light exposure and substantially lower evening and nighttime exposure help create a stronger distinction between day and night.
The same spectral power distribution therefore cannot be evaluated independently of the time and duration of exposure.
Spatial Distribution Matters
Traditional lighting calculations often concentrate on horizontal illuminance at a desk or floor.
Circadian lighting adds another important consideration: light at the eye.
Windows, illuminated walls, coves, indirect lighting, luminous ceilings, and other large surfaces can contribute to vertical illumination differently from luminaires designed primarily to illuminate the horizontal workplane.
This is why circadian performance cannot be determined from CCT or luminaire wattage alone.
Circadian Lighting Metrics: Melanopic EDI, m-DER, EML and CS
Different frameworks use different methods to quantify non-visual light exposure.
They are related, but they should not be treated as interchangeable.
Melanopic EDI
Melanopic Equivalent Daylight Illuminance (melanopic EDI) is a CIE-defined quantity for describing melanopsin-weighted light exposure.
It expresses a given melanopic stimulus as the illuminance of standard daylight that would provide the equivalent melanopic response.
Melanopic EDI is measured in lux and has become one of the principal quantities used in contemporary circadian-lighting research.
For architectural applications, it is particularly useful because it connects the spectrum of the source with the actual illuminance reaching the eye.
Melanopic DER
Melanopic Daylight Efficacy Ratio (melanopic DER or m-DER) describes the melanopic effectiveness of a spectrum relative to its conventional photopic effectiveness.
It is useful when comparing two light sources.
For example, two LEDs may produce the same photopic illuminance but have different melanopic DER values because their SPDs are different.
However, melanopic DER describes a characteristic of the spectrum — not the total exposure.
Actual melanopic EDI still depends on the amount of light reaching the eye.
Equivalent Melanopic Lux
Equivalent Melanopic Lux (EML) is an earlier melanopic metric that remains relevant because it has been used within the WELL Building Standard.
EML and melanopic EDI use different normalization conventions. They are related mathematically, but designers should use the metric specified by the applicable standard or project requirement rather than casually switching between them.
Circadian Stimulus
Circadian Stimulus (CS) is a separate model developed by the Lighting Research Center.
It has also been used in design guidance such as UL DG 24480.
CS is based on a different model from melanopic EDI and EML, so values from these systems should not be mixed as though they represented the same measurement.
For professional specifications, the first question should therefore be:
Which metric and design framework is this project actually using?
Circadian Lighting Design Targets
There is no universal circadian lighting schedule that is correct for every person, building, and application.
However, an international expert consensus published in 2022 proposed useful reference values for healthy adults with regular daytime schedules:
Daytime: at least 250 lx melanopic EDI at the eye throughout the daytime
Evening: no more than 10 lx melanopic EDI during approximately the three hours before habitual bedtime
During sleep: ideally below 1 lx melanopic EDI
These should be understood as science-based recommendations rather than universal building-code requirements.
They were developed primarily for healthy adults aged approximately 18–55 with conventional daytime schedules. Children, older adults, shift workers, patients, and other populations may require different considerations.
The more useful design principle is therefore not a single number but a relationship:
stronger daytime light exposure + substantially reduced evening and nighttime exposure.
Circadian Lighting and WELL
WELL includes circadian-lighting criteria intended to support appropriate light exposure for building occupants.
Depending on the applicable WELL version, project type, pathway, and feature requirements, designers may need to evaluate melanopic exposure at the eye over defined periods of the day.
These requirements evolve as the underlying science develops, so project teams should always verify the current WELL requirements rather than relying on a fixed EML value quoted from an older project or publication.
Most importantly, WELL circadian-lighting requirements cannot be satisfied merely by specifying a high-CCT fixture.
Successful design depends on the interaction between:
spectral power distribution
illuminance at the eye
spatial distribution
daylight contribution
occupant position
exposure duration
control strategy
For projects pursuing certification, the selected lighting system should provide sufficient spectral and photometric data for the project team to perform the required calculations.
Circadian Lighting Design: Five Principles
1. Create a Meaningful Day-to-Evening Transition
Circadian lighting should create a clear distinction between daytime and evening conditions rather than simply moving slowly between arbitrary CCT values.
During the day, a design may prioritize higher vertical illumination and a spectrum appropriate for daytime exposure.
Toward evening, overall intensity and melanopic exposure can be progressively reduced.
The visible change in CCT may be dramatic in some environments and subtle in others.
There is no requirement that every circadian system follow a fixed progression such as 6500K → 4000K → 2700K. The appropriate sequence depends on the architecture, occupants, daylight availability, use of the space, and desired experience.
2. Design for Light at the Eye
A beautiful 500-lux workplane does not necessarily mean that a useful amount of light reaches the eye.
The geometry of the space matters.
Coves, illuminated walls, luminous ceilings, indirect systems, and daylight openings can all contribute to vertical illumination. Downlights and task lights can also contribute, but their effectiveness depends on beam distribution, position, surface reflectance, and viewing direction.
Rather than assuming that one luminaire type is inherently “more circadian” than another, designers should evaluate the actual eye-level exposure created by the complete lighting system.
Linear LED is particularly useful in this context because it can be integrated into coves, ceilings, walls, architectural profiles, and other large luminous elements.
Explore Yuji Lux linear lighting solutions.
Vertical illuminance at the eye: the 7:10 rule
Circadian metrics — melanopic EDI, EML, CS — are specified vertically at the eye, on a plane facing the occupant's viewing direction at roughly 1.2 m above the floor for a seated person. Conventional lighting design targets horizontal illuminance on the work plane, and the two are not the same number. Under ceiling-mounted lighting the vertical illuminance at the eye is considerably lower than the desk value: a commonly cited planning ratio for open-plan offices is about 0.7 (some office studies put the practical ceiling nearer 0.6).
A worked example: an office designed to 500 lx on the desk delivers about 350 lx vertically at the eye at a 0.7 ratio. With a standard 4000K LED at a melanopic DER of about 0.65, that is 350 × 0.65 ≈ 230 lx melanopic EDI — below the 250 lx daytime reference of Brown et al. 2022, although the visual design is fully compliant. With a daylight-oriented spectrum whose melanopic DER is close to 1, the same 350 lx at the eye gives roughly 350 lx melanopic EDI. The spectrum, not more lux, closed the gap.
The rule: predict or measure at the eye, treat the desk value as an upper bound, and raise vertical illuminance with illuminated walls, coves and daylight openings before adding raw brightness. The definition and formula are in What is melanopic EDI?.
3. Specify Spectrum, Not Only CCT
CCT is useful for describing the appearance of nominally white light, but it does not uniquely define a spectrum.
Two 3000K, 4000K, or 5000K sources can have substantially different SPDs.
For projects where circadian performance matters, request:
Spectral Power Distribution (SPD)
melanopic DER or equivalent melanopic data
photometric performance
CRI and R9
TM-30 data where appropriate
chromaticity and color consistency
performance at relevant tuning and dimming points
This is especially important for Tunable White systems.
A two-channel system creates intermediate spectra by combining its warm and cool channels. The resulting SPD, color rendering, Duv, melanopic performance, and output can therefore vary throughout the tuning range.
Endpoints alone do not describe the entire system.
For a deeper explanation of why CCT and CRI cannot fully characterize a light source, see Color Consistency and Spectral Lighting Design: Why CCT and CRI Are Not Enough.
4. Protect the Evening
Circadian lighting conversations often focus on increasing daytime exposure.
The evening deserves equal attention.
Reducing CCT can be useful, but warm appearance does not automatically mean low melanopic exposure.
The final melanopic exposure depends on both spectrum and intensity.
A conventional 2700K LED at a sufficiently low level may work perfectly well in many evening environments. At higher required light levels, a spectrum specifically engineered for reduced melanopic content can provide another design tool.
This can be particularly valuable in:
bedrooms
senior living
wellness environments
hospitality
residential circulation areas
late-evening lounges
other spaces used close to bedtime
Yuji's FlameWarm™ technology is one example of this spectral approach.
5. Preserve Visual Quality
Circadian performance is not a substitute for good lighting design.
A system must still provide appropriate:
visual comfort
color rendering
skin-tone rendering
chromaticity consistency
glare control
flicker performance
dimming quality
architectural integration
Optimizing a biological metric while producing unattractive or uncomfortable light is not a successful Human-Centric Lighting strategy.
The goal is to integrate visual and non-visual requirements into the same environment.
Circadian Lighting by Application
Healthcare and Senior Living
Healthcare and senior living are important applications for circadian lighting because occupants may spend extended periods indoors and may have limited access to daylight.
A practical strategy is to create substantial daytime light exposure in patient rooms and common areas while maintaining carefully controlled evening and nighttime conditions.
Studies in care settings have reported improvements in outcomes including daytime activity, agitation, mood, and, in some cases, sleep when daytime light exposure was increased.
However, older adults can respond differently to the same measured environmental light exposure because age-related changes to the eye affect the amount of light reaching the retina.
For this reason, healthcare and senior-living projects should be designed specifically for the target population rather than applying office-lighting targets without modification.
Offices and Workplaces
Workplaces can combine daylight and electric light to provide appropriate daytime exposure.
Rather than specifying cool-white fixtures simply because they are assumed to be more “energizing,” designers should consider:
daylight availability
vertical illuminance
SPD
occupant orientation
workstation position
glare
duration of exposure
controls
Circadian performance should be evaluated alongside traditional workplace requirements such as visual comfort, energy use, task illumination, and architectural quality.
Hospitality and Restaurants
Hospitality often presents almost the opposite design challenge.
Restaurants, lounges, hotels, and wellness spaces may primarily operate during the late afternoon and evening, when maximum circadian activation is usually not the objective.
The designer may instead aim to reduce melanopic exposure while maintaining enough light for circulation, dining, material appreciation, facial recognition, and visual comfort.
This makes spectral engineering particularly useful: the system can become visually warm and biologically lower-stimulus without necessarily reducing the entire space to extremely low light levels.
Residential Lighting
Residential circadian lighting should respond to the actual schedule of the occupants.
During the daytime, living spaces can make maximum practical use of daylight supplemented by electric light.
During the evening, living rooms, bedrooms, bathrooms, and circulation areas can transition toward lower intensity and lower melanopic exposure.
The best system is often one occupants barely need to think about.
Automated scenes can establish the daily progression while still allowing manual control when needed.
Daylight: The Starting Point for Daytime Circadian Lighting
Whenever it is practically available, daylight should be considered the first source of daytime circadian exposure.
Daylight can provide high illuminance together with a naturally changing spectrum, but its contribution to an interior varies enormously with:
orientation
glazing
latitude
season
time of day
cloud cover
shading
façade design
surface reflectance
distance from the window
This makes simple rules such as “everything within X feet of a window receives enough daylight” unreliable.
Daylight contribution should be evaluated for the actual project.
Electric circadian lighting should therefore work with daylight rather than independently from it.
A useful design concept is:
Daylight when available + electric light when needed + controls that coordinate both.
Electric lighting becomes especially important during early occupancy, overcast conditions, in deep floor plates, shaded interiors, and at times when natural daylight is unavailable.
Tunable White Is a Technology — Not a Circadian Strategy
Tunable White is often marketed as synonymous with Human-Centric or circadian lighting.
It is not.
Tunable White simply means that a lighting system can change its white-light chromaticity, typically by mixing two or more LED channels.
That capability can be extremely useful for circadian design, but it does not prove that the system provides an appropriate biological light exposure.
A Tunable White system still needs to be evaluated for:
SPD across the tuning range
melanopic performance
light level at the eye
timing
daylight contribution
control behavior
visual quality
Conversely, not every circadian strategy requires continuous Tunable White.
A project could use daylight, static daytime lighting, and a separate low-melanopic evening source and still create a highly effective day-to-evening lighting strategy.
The design intent comes first.
The technology follows.
Controls and Programming
Circadian lighting is inherently connected to time, which makes controls an important part of the system.
Even an excellent spectrum will not produce the intended daily pattern if it is delivered at the wrong time or at the wrong intensity.
A control sequence may include:
morning transition
daytime mode
afternoon transition
evening mode
low-level nighttime mode
The actual sequence should reflect building use rather than a generic astronomical schedule.
An office, restaurant, hospital, residence, and hotel room do not need the same program.
Control platforms may include DALI, 0–10V, DMX, Lutron, Casambi, or other systems depending on the application.
More important than the protocol itself is coordination between:
LED source → driver → controller → programmed scene → final light at the eye.
The entire chain should be evaluated as one system.
Manual controls are also important. Occupants need flexibility, but automated scheduling can provide the baseline behavior without requiring users to manually recreate the intended circadian cycle every day.
Specifying a Circadian Lighting System
Circadian lighting should be specified at the system level rather than by choosing an LED product with “human-centric” printed on the datasheet.
Depending on the project, useful specification information may include:
Spectral Power Distribution (SPD) at relevant operating points
Melanopic DER or other required melanopic data
Melanopic EDI calculations for representative occupant positions
Photometric data and vertical illuminance calculations
CRI and R9
TM-30 data where color fidelity is important
Chromaticity and color consistency
Duv behavior across the tuning range
Flicker performance
Dimming range and low-end behavior
Driver and control compatibility
Daylight contribution
Commissioning and field-measurement strategy
For Tunable White products, important characteristics should be verified at representative intermediate settings rather than only at the warmest and coolest endpoints.
The luminaire and architectural assembly matter as well.
Diffusers, lenses, profiles, reflectors, surface finishes, thermal conditions, and installation geometry can all influence the final light delivered into the space.
At Yuji Lux, spectral measurements can be evaluated at the complete system level, including the effects of profiles and diffusers where required. Learn more about how we work.
How Yuji Lux Approaches Circadian Spectral Design
At Yuji Lux, circadian lighting begins with spectral engineering rather than CCT labels.
The objective is not to create one supposedly ideal “circadian LED.” Different periods of the day call for different lighting conditions.
For daytime applications, the goal may be to create high-quality, daylight-oriented illumination capable of supporting meaningful daytime melanopic exposure while maintaining excellent color rendering and architectural appearance.
For evening applications, the design direction changes: melanopic exposure can be deliberately reduced while preserving useful illumination and visual quality.
This is why Yuji uses different spectral tools for different parts of the day.
SunWave™ for Daylight-Oriented Lighting
SunWave™ Dynamic White is a 2700–6500K full-spectrum dynamic-white LED system designed for applications requiring daylight-oriented spectral quality and smooth white-light tuning.
Rather than treating Tunable White purely as a CCT-changing effect, SunWave™ is designed around spectral quality throughout the usable range.
The current SunWave LED strip provides CRI 98, R9 90, TM-30 Rf/Rg 98/100, and a dynamic 2700–6500K range.
It can be integrated into architectural profiles, coves, indirect lighting, wellness environments, and daylight-simulation applications.
FlameWarm™ for Low-Melanopic Evening Lighting
For evening conditions, Yuji takes a different spectral approach.
Dim-to-FlameWarm™ transitions from 3000K to approximately 1250K as it dims.
At its warmest operating point, FlameWarm™ achieves a very low melanopic-to-photopic ratio of approximately 0.099, while retaining broadband spectral content and high color-rendering performance.
This makes it a useful tool when a project requires both an ultra-warm evening atmosphere and deliberately reduced melanopic content.
The objective is not to claim that every evening environment requires a specialized spectrum.
Instead, FlameWarm™ gives designers an additional degree of freedom where conventional warm-white lighting would require very low illuminance to achieve the desired melanopic target.
From Products to a Complete Circadian Lighting System
A successful circadian lighting project cannot be reduced to selecting a single LED strip.
The final result depends on the interaction between:
Spectrum × Intensity × Timing × Spatial Distribution × Daylight × Controls
The LED spectrum establishes the starting point.
The profile and diffuser modify the emitted light.
The architecture determines where that light travels.
The control system determines when and how it is delivered.
And the commissioning process verifies what occupants actually experience.
That system-level approach is more useful than simply labeling a product “circadian.”
Yuji Lux works with architects, lighting designers, integrators, and manufacturers to evaluate complete linear lighting assemblies and develop project-specific spectral strategies.
Frequently Asked Questions
What is circadian lighting?
Circadian lighting is a lighting approach that considers how light exposure interacts with the human circadian system throughout the day.
It evaluates factors including spectrum, intensity, timing, duration, and light at the eye, rather than relying only on CCT or horizontal illuminance.
What is Human-Centric Lighting?
Human-Centric Lighting, or HCL, is a broad term for lighting approaches intended to consider human visual comfort, experience, performance, and non-visual responses to light.
Circadian lighting is more specifically concerned with the relationship between light exposure and the body's approximately 24-hour biological rhythms.
Because the terminology is used inconsistently in the lighting industry, professional specifications should define measurable performance requirements rather than relying on the HCL label alone.
Is Tunable White the same as circadian lighting?
No.
Tunable White is a technology that allows the white-light appearance of a source to change.
Circadian lighting is a design strategy.
Tunable White can be part of that strategy, but changing CCT alone does not demonstrate circadian performance.
Spectrum, intensity, timing, duration, spatial distribution, daylight, and eye-level exposure must also be considered.
What is melanopic EDI?
Melanopic Equivalent Daylight Illuminance is a CIE-defined quantity used to characterize melanopsin-weighted light exposure.
It is expressed in lux and allows designers and researchers to evaluate the melanopic component of the light actually reaching the eye.
What is melanopic DER?
Melanopic Daylight Efficacy Ratio describes how melanopically effective a spectrum is relative to its photopic effectiveness.
It helps compare spectra, but it does not describe actual exposure by itself.
The actual melanopic EDI also depends on illuminance at the eye.
How much melanopic light is recommended during the day?
A 2022 international expert consensus recommends at least 250 lx melanopic EDI at the eye during the daytime for healthy adults with regular daytime schedules.
The same consensus recommends substantially lower exposure during the evening and sleep period.
These values are scientific recommendations rather than universal building-code requirements, and specific populations may require different considerations.
Does circadian lighting meet WELL requirements?
Circadian-lighting strategies can contribute to WELL lighting requirements, but there is no single product or CCT setting that automatically makes a project WELL compliant.
WELL uses project-specific criteria that can include melanopic exposure, duration, spatial coverage, and other requirements.
Because WELL requirements are periodically updated, designers should use the current version of the applicable feature for certification calculations.
Is blue light at night harmful?
The issue is more nuanced than saying that “blue light is harmful.”
Short-wavelength light is particularly effective at stimulating the melanopic system, and sufficiently high light exposure during the evening can influence melatonin and circadian timing.
However, the response depends on spectrum, intensity, duration, timing, and individual sensitivity.
The practical lighting-design objective is therefore not to eliminate blue wavelengths entirely, but to control inappropriate melanopic exposure during the evening while maintaining safe and visually comfortable lighting.
Does warm light automatically have low circadian impact?
No.
Lower-CCT light often has lower melanopic content than cooler light, but CCT alone cannot determine the melanopic exposure.
Two sources with the same CCT can have different SPDs and different melanopic DER values.
Intensity matters as well.
A low-melanopic spectrum used at high illuminance may still create meaningful melanopic exposure, while a conventional warm-white source used at sufficiently low illuminance may already meet the desired evening target.
What makes an LED strip suitable for circadian lighting?
There is no LED-strip construction that automatically makes a product circadian.
A suitable system should provide documented spectral and photometric performance, stable color quality, appropriate control capability, and enough output and flexibility to achieve the required eye-level exposure.
Linear LED is particularly useful because it can be integrated into coves, ceilings, walls, profiles, millwork, and large luminous surfaces.
Do circadian lighting systems need to change color throughout the day?
Not necessarily.
Dynamic white tuning can be useful, but the most important objective is the daily pattern of light exposure.
Some projects may use continuous spectral tuning. Others may combine daylight, a static daytime source, and a separate low-melanopic evening source.
The best approach depends on the project.
Can daylight be included in circadian lighting calculations?
Yes.
Daylight can make a major contribution to daytime eye-level exposure and should be considered together with electric lighting.
Its contribution varies with façade geometry, orientation, glazing, shading, season, weather, and occupant position, so it should be evaluated for the specific project rather than assumed.
Planning a Circadian Lighting Project?
Yuji Lux works directly with architects, lighting designers, integrators, and manufacturers to define spectral requirements and develop complete linear lighting systems.
Whether the project requires daylight simulation, Tunable White, low-melanopic evening lighting, custom spectral engineering, or integration with an existing control platform, our team can help evaluate the full system — from SPD and LED selection to profiles, diffusers, drivers, controls, and final optical performance.
Explore Circadian Lighting Solutions
Explore SunWave™ Dynamic White
Read the Professional Guide to Circadian Lighting & HCL
Discuss Your Project with Yuji Lux
References
Brown TM, Brainard GC, Cajochen C, Czeisler CA, Hanifin JP, Lockley SW, et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLOS Biology. 2022.
CIE S 026/E:2018. CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light. International Commission on Illumination.
WELL Building Standard. Circadian Lighting Design. International WELL Building Institute. Refer to the current applicable WELL version and feature requirements for project-specific thresholds.
UL Design Guideline 24480. Design Guideline for Promoting Circadian Entrainment with Light for Day-Active People.
This guide is the technical foundation of our Human-Centric & Circadian Lighting area — the design framework, day-period reference guidance, tools, and spectral platforms are collected there.
Selecting sources? See Tunable White LED strips for circadian lighting — how to evaluate a circadian Tunable White strip and where SunWave Dynamic White fits.