LEED, WELL and BREEAM: Standards, Protocols and Certifications for Light Design
Guidelines and design requirements for integrating natural light, visual comfort and biological effects into environmental sustainability protocols
Human-Centered Lighting (HCL) represents an advanced design paradigm that integrates the classic criteria of lighting technology (visibility, energy efficiency, aesthetics) with the most recent knowledge on the non-visual effects of light, in particular on the human circadian system. The growing attention to psychophysical well-being in built environments has led to the development of technical standards and voluntary certifications that offer concrete tools to integrate these principles into design practice. At the regulatory and certification level, documents such as DIN/TS 67600, the WELL Building Standard, the LEED system and the BREEAM protocol now provide fundamental references for health-oriented lighting design. These tools define metrics, light exposure targets, and design criteria to ensure that light – natural or artificial – actively supports circadian rhythm, visual functionality, and environmental comfort.
In the international arena, Germany was a pioneer with DIN SPEC 67600:2013, a technical specification that provides guidelines for the design of biologically effective lighting. This document (recently evolved into DIN/TS 67600:2022) suggests practical values and criteria: for example, it recommends ensuring a certain minimum level of vertical illuminance to the eye in the morning hours to synchronize the circadian rhythm. In particular, a vertical illuminance ≥ 250 lux of daylight-equivalent light (at high color temperature, ~6500 - 8000 K) on the user's eye for at least 4 hours in the morning is indicated as a target. This level serves to almost completely suppress melatonin during the early hours of the day and to increase the level of "daytime" cortisol, so that people are awake and alert. These recommendations have influenced subsequent standards and international guidelines such as the WELL protocol. Although it is a voluntary specification and not a mandatory standard (as the acronym SPEC indicates), DIN 67600 is frequently cited as an authoritative technical reference in Europe, in particular for design oriented towards the non-visual effects of light. The document also provides guidance for the rest of the day, suggesting bright, blue-rich light in the middle of the day, and a gradual decline in intensity and color temperature in the evening hours. For example, between 6:00 p.m. and 8:00 p.m., a relaxing light with about 200 lux on the eye and a CCT of no more than 3000 K is recommended. When designing according to these guidelines, it is essential not to neglect the visual requirements: DIN SPEC 67600 calls for compliance with current lighting standards such as DIN EN 12464-1, emphasizing that circadian light must be provided in addition to the minimum visual standards, and not as a substitute for them. This involves, for example, the integration of additional vertical lighting or the use of dimmable sources that offer both visual support and biological stimulation.
Although published in 2013, DIN SPEC 67600 anticipated many key concepts now integrated into documents from the CIE, the Lighting Research Center and the WELL Building Standard, and still remains a fundamental reference for Human-Centric design in Europe.
The WELL Building Standard (Light Concept) focuses on the well-being of the occupants, dedicating an entire area to lighting parameters. In addition to information on light quality (colour rendering, flicker, etc.) and glare control, WELL introduces pioneering requirements in the field of circadian lighting. In particular, Feature 54: Circadian Lighting Design requires that a sufficient level of effective light in the workplace is guaranteed on the circadian level. For example, at least 75% of workstations must receive ≥200 lux melanopic equivalent (EML) vertically at eye level, between 9:00 and 13:00 each day. This value (200 EML) is calculated by weighting the light spectrum according to the sensitivity of melanopsin, so it represents the biologically "active" component of light. At the same time, WELL requires limiting night light to low melanopic levels: for example, in bedrooms or residential environments, melanopic illuminance must not exceed 50 EML at night, so as not to interrupt sleep. In addition to circadian criteria, the WELL standard promotes natural light through specifications such as Right to Light (which requires a high percentage of occupants to sit near windows) and Daylight Modeling, prompting designers to optimize daylight intake in the design phase. The overall aim is to create indoor environments that reduce circadian disruption, improve sleep quality and mood, and increase users' visual comfort.
At the same time, LEED certification (mainly oriented towards energy and environmental sustainability) does not yet include credits explicitly dedicated to circadian lighting, focusing more on energy efficiency and daylighting aspects (e.g. maximizing natural light to reduce electricity consumption).
LEED (Leadership in Energy and Environmental Design) is a widespread environmental certification system that includes, in the Indoor Environmental Quality category, credits dedicated to natural lighting and indoor light quality. The LEED v4/v4.1 Daylight credit strongly encourages daylight design with the aim of "connecting occupants with the outdoors, reinforcing circadian rhythms and reducing the use of electric lighting". LEED requires demonstrating, through annual simulations, that a significant portion of occupied spaces receive useful levels of natural illuminance. For example, to obtain 2 points, the project must achieve a Spatial Daylight Autonomy sDA300.50% of at least 55% (i.e. at least 55% of the occupied area enjoys ≥300 lux during the day for at least 50% of the annual hours of use), and up to 3 points if at least 75% is reached. At the same time, solar glare must be controlled: the Annual Sunlight Exposure criterion limits to 10% the share of the area that can receive more than 1000 lux for over 250 hours/year, requiring solutions such as shading or solar control glass to mitigate excess direct sun. Alternatively, LEED allows an instrumental verification on two dates (September equinox at 9:00 and 15:00) by imposing that at those times all rooms have illuminances between 270 and 5,400 lux in the absence of artificial lights. In addition, it is mandatory to provide glare control systems (screens, curtains, adjustable sunshades) for all windows in occupied spaces, ensuring visual comfort. LEED also considers the view to the outside (Quality Views credit) as a component of well-being: designing with large transparent glass surfaces and views of the landscape improves user satisfaction and health. Overall, the LEED requirements direct designers to buildings with extensive use of diffused natural light, interior layouts that bring daylight deep into the depths (e.g. atriums, skylights, interior glass walls) and devices to avoid both light deficiencies and glare excesses. The focus on circadian rhythms is explicit in the intent of the Daylight credit: this represents a recognition, in the field of sustainability, of the link between daylight, human well-being and performance.
The difference in approach between WELL and LEED is pointed out by experts: while LEED uses daylight as an energy-saving resource, WELL considers it above all to stimulate the human circadian rhythm; moreover, WELL encourages the use of dynamic electric light (tunable white) for health, an aspect not addressed by LEED. It is plausible that future versions of LEED will integrate human-centric lighting aspects as these practices become part of the design "mainstream". In the meantime, designers and clients interested in well-being can pursue dual certification, using HCL as a key element to obtain WELL points without contravening LEED requirements (e.g. by ensuring high circadian levels during the day but with efficient dimmable systems so as not to penalize consumption).
Unlike other certifications, such as LEED or WELL, which have a stronger focus on energy sustainability or user health, BREEAM offers a balanced approach, assessing a building's performance holistically.
The BREEAM (Building Research Establishment Environmental Assessment Method) is one of the most widely used and recognised environmental certification systems for buildings at an international level. Introduced in 1990 by the BRE (Building Research Establishment) in the United Kingdom, BREEAM assesses and certifies the environmental sustainability of buildings according to rigorous and multidimensional criteria, covering aspects such as energy efficiency, the use of sustainable materials, indoor environmental quality and occupant comfort. Similar to LEED, BREEAM includes a Health & Wellbeing section in which lighting plays a key role. The HEA 01 – Visual Comfort requirement covers various aspects: illuminance, glare, external view and user control. With regard to natural light, BREEAM adopts the criterion of the average daylight factor (DF): to obtain full credit, it is required that at least 80% of the surface area of the occupied rooms has a daylight factor ≥2%. In other words, almost all the space used regularly must receive an amount of natural light equal to at least 2% of the diffused outdoor light (measured on an overcast sky). This target ensures that rooms are generally well lit during the day. (In residential settings, BREEAM prescribes similar values: e.g. kitchens with DF ≥2%, living rooms ≥1.5%). In addition to the level of natural lighting, BREEAM assesses the presence of a view to the outside (e.g. at least 95% of the spaces must have a window with a view to the outside at a distance of within 7 m) and glare control measures (fixed or mobile shading for windows exposed to direct sun). Individual adjustment is also rewarded: providing occupants with the possibility of adjusting the artificial lighting in their areas (dimmer, zone control) to adapt it to their needs. The goal is to ensure optimal visual comfort in all conditions: enough natural light to carry out activities during the day, while avoiding both dark areas and glare phenomena or excessive contrasts that strain the eyes.
The European standard UNI EN 12464-1:2021, which regulates the lighting requirements for indoor workplaces, has also introduced an important conceptual update in its most recent revision, explicitly recognizing the role of light not only for functional visual vision, but also for the regulation of biological rhythms and the influence on the psychophysical well-being of users. Although it remains a norm focused on minimum visual lighting requirements – such as average illuminance on the task, uniformity, controlled glare (UGR) and colour rendering (Ra) – it integrates for the first time qualitative and informative indications related to the circadian and neuroendocrine effects of light. Such effects, often referred to as non-visual effects or biological effects of light, depend mainly on the stimulation of melanopsin in light-sensitive retinal ganglion cells (ipRGCs), which are sensitive to light rich in blue–cyan components (about 460–490 nm). The standard does not introduce specific quantitative requirements, but encourages the designer to take a holistic and integrated approach, taking into account the temporal, spatial and spectral parameters of artificial lighting. In particular, EN 12464-1:2021 highlights how the total amount of light received by the eye, the angle of incidence, the correlated color temperature (CCT), the spectral distribution of the source and the time of light administration are all determining factors in generating an effective circadian response. The importance of increasing vertical illuminance on the eye, especially in the early hours of the day, to improve the synchronization of the internal biological clock, is therefore emphasized.
This approach is conceptually close to what is proposed by DIN SPEC 67600 and the WELL protocols, while maintaining a more open and less prescriptive formulation. In addition, the standard invites to evaluate the combination of natural and artificial light, emphasizing how a well-designed lighting system must support not only visual performance, but also long-term psychophysiological well-being. This integration is also reflected in the quality of light distribution in space, in the balance between direct and indirect light, and in the possibility of individual control of lighting by users, all of which are correlated with circadian efficacy and perceptual comfort. From an application point of view, EN 12464-1:2021 does not impose minimum thresholds of melanopic lux or other biological indices, but formally opens up the possibility that these parameters are considered in the advanced design phase, especially in environments where comfort and vigilance are critical, such as schools, offices, hospitals or centers for the elderly. In this sense, the standard represents a cultural turning point: lighting is no longer seen as a mere technical tool to ensure visibility, but as an active component of the built environment, capable of modulating human behavior, mood and health. This orientation is consistent with the evolution taking place at an international level, in which dynamic light, Tunable White systems, circadian controls and biological metrics are progressively integrated into simulation software and competition specifications for the creation of high-performance indoor environments. Ultimately, EN 12464-1:2021, while not yet a binding tool in the circadian field, provides the lighting designer with an up-to-date technical and cultural framework, encouraging the transition to human-centered design, based on scientific data and real well-being goals, in line with the most advanced international standards.
Finally, as already described in the previous chapters, scientific communities and professional associations (such as the CIE – Commission Internationale de l'Éclairage and the IESNA – Illuminating Engineering Society of North America) are contributing substantially to the development of specific metrics and guidelines to support a truly human-centered lighting design, based on neurophysiological and chronobiological evidence. Among the most relevant innovations that have emerged in recent years is CIE S 026:2018, which introduced a five-channel photometric system based on the spectral response of the main retinal photoreceptor populations: S, M, L cones, rods and intrinsically photosensitive ganglion cells (ipRGCs) containing melanopsin. This system allows to evaluate with scientific rigor the non-visual impact of light, in particular on the regulation of circadian rhythms. Within this regulatory context, two fundamental metrics have been defined:
EML – Equivalent Melanopic Lux, adopted by the WELL Building Standard v2, calculated as the ratio between melanopic and photopic illuminance with reference to a standard source (D65). The EML metric, already incorporated into the WELL Building Standard protocol, represents an operational value directly comparable with circadian performance thresholds, useful for calculating the efficacy of light in real environments through spectrophotometric data and simulation software;
MEDI – Melanopic Equivalent Daylight Illuminance, expressed in melanopic lux equivalents, which today represents the official metric recommended by the CIE for the evaluation of artificial light-induced circadian stimulation. The MEDIUM, on the other hand, offers a more advanced normalization: it expresses theequivalent illuminance in daylight (D65 spectrum) which would have the same stimulating effect on melanopsin as the source analyzed. This approach allows to establish a Unified and comparable reference for biological evaluations, freeing itself from the variability of artificial sources and offering a criterion consistent with the natural lighting conditions. The CIE recommends expressing circadian levels in MEDI, as it allows for a more rigorous and comparable application between projects, overcoming some ambiguities present in the direct use of melanopic lux equivalents.
Both metrics derive from a spectral weighting of photopic illuminance as a function of melanopsin sensitivity, a photopigment with peak absorption at about 480 nm, crucial for the neuroendocrine and chronobiological regulation of the human body.
Alongside these two physical-photometric metrics, there is the CS – Circadian Stimulus parameter, proposed by the Lighting Research Center (LRC). Unlike EML and MEDI, CS is based on a non-linear physiological model and describes the relative effect of melatonin suppression as a percentage of a maximum reference condition (generally 1000 lux of white light rich in blue component). The CS takes into account threshold, saturation and logarithmic response phenomena of the endocrine system, being particularly adherent to experimental chronobiology but more complex to calculate.
From a design point of view, the AVERAGE, EML and CS metrics are now essential tools for the scientific evaluation of the non-visual effectiveness of a lighting scheme. These indicators allow lighting designers to overcome the limitations of conventional approaches, which are still often based exclusively on parameters such as photopic lux, correlated color temperature (CCT) or color rendering index (Ra). On the contrary, the adoption of these metrics introduces an integrated view in which factors such as vertical illuminance on the eye, spectral composition of light, exposure time and directionality of luminous flux play a decisive role in ensuring effective circadian stimulation. The technical definition, the neurophysiological basis and the calculation criteria of each of these metrics are explored in depth in the chapter dedicated to "Fotopic vs Melanopic Illuminance: Definition and Spectral Sensitivity", which analyzes with scientific rigor the fundamentals, methodological differences and design implications. This theoretical framework is essential for the correct use of melanopic values within simulation software, certification protocols (such as the WELL Building Standard) and performance specifications of modern Human-Centric Lighting systems.
This new frontier of design, strongly supported by neuroscientific evidence, marks an important step forward towards built environments that are biologically synchronized with the natural rhythms of the human body, contributing to the improvement of alertness, mood, sleep quality and, in general, the well-being of users.
