Circadian Lighting? – you’re asking the wrong question

We’ve recently seen the term Human Centric Lighting (HCL) be dropped in favour of the apparently more focused ‘circadian lighting’. But this simply maintains an assumption that HCL should only relate to the direct physiological impact that lighting can have on the body’s systems. And it hasn’t helped that HCL has been hijacked by manufacturers’ marketing departments to promote exactly that narrow view of what HCL is about; it doesn’t address the subject in its holistic entirety.

Wellness in the Workplace

A better term that’s coming into its own is ‘Wellness’, arriving chiefly from the work of the WELL Building Institute and its WELL Standard. This is a more useful description because it broadens the idea of what’s being sought. In terms of the workplace, ‘circadian lighting’ can only be about circadian entrainment; it’s the desire to help support the body to maintain a steady physiological rhythm throughout the 24-hour day. But the concept of ‘Wellness’ is bigger than that.

We’re able to identify three drivers in achieving Wellness through lighting:

  • physiological
  • psychological
  • emotional

Physiologically, we’ve always worked from an intention to  ‘do no (physical) harm’. Every lighting code that’s ever been produced is based on the ability to achieve a task safely and efficiently. That pre-supposes a physiological Good. When we add circadian criteria to that intent then we are simply expanding on the role that lighting plays in ‘doing no harm’. There are plenty of examples of people working in environments that deny them access to natural light and where current lighting practice is almost certainly ‘doing harm’; industrial and transport environments that are cut-off from daylighting; geographical zones where the natural rhythm of the day is at odds with the working day, such as polar stations; and all shift working, of course.

Psychology plays an enormous part in our lives. One of the outcomes of early HCL schemes in education demonstrated a psychological effect rather than a physiological one. The more interesting examples came out of schools in Scandinavia. These involved lighting installations where teachers had manual control of the lighting. Depending on the classroom activity, the teacher could adjust light tone and illuminance levels. That manual control tended towards bright and cool white lighting to aid concentration, and lower levels of warm light for relaxation – storytelling sessions, for example.

Teachers reported immediate benefits for the children. These behavioural responses were appropriated by the proponents of ‘circadian lighting’ – but this could not have been the case. The body’s hormonal systems do not work that quickly – but the brain does. It is far more likely that those changes in lighting states triggered a psychological response rather than a physiological one.

Emotion is what’s behind the move towards Biophilic Design. This is a ‘does-the-heart-good’ approach to the visual environment. The biophilic movement identifies three levels of nature connection:

A biophilic analogue dessert from The Elderflower, Lymington
  • Direct access to nature using visual engagement, such as creating positive views out of a building and having living planting inside a building – and working with other sensory inputs; sounds of nature, gentle air movements and so on,
  • Representations of nature (analogues) such as nature images in the form of 2D and 3D artworks, using ‘natural’ building materials, textures and patterns, etc,
  • Employing spatial design that removes the tendency towards geometric arrangements and logical order in human construction – making working spaces ‘organic’ in their layouts.

How do we create a lighting brief for such intangible factors as these? Do we have to start from scratch or are the sign-posts already in place to assist us in that journey?

Let’s go through those three factors again and see what we might be working with:

Physiology: if we work outwards from the ‘first, do no harm’ position, we see that much of the work has been done for us. Lighting Codes of Practice provide us with all of the task-based data that we need to apply; issues around visual acuity and comfort, such as glare and flicker, have applicable limiting factors, as in any project.

The circadian issue is not yet resolved. Illuminance values have been cited by the WELL Standard and by Codes of Practice such as German DIN SPEC 67600 (see the table below). Studies still need to be carried out over a long period of time before any certainty can be brought to bear to this situation.

What we can state with some certainty is that – for the workplace – higher illuminance levels are required in the first hours of the working day in order to ensure circadian entrainment by the body’s hormonal systems. Its likely that, for most people, this is only an issue during the winter months – and then it can be a critical factor. If we insist on working the same hours every working day throughout the year (its not a condition of being, its just a choice that we make) then we need to do better for all building occupants to support the body’s systems at those times.

Three factors need to be addressed when creating a circadian lighting brief:

  • The amount of vertical illuminance that meets the eye, measured in lux of melanopic daylight equivalent illuminance or in equivalent melanopic lux (EML)),
  • The quality of that light in terms of colour temperature and colour rendering; whether its a fixed colour temperature throughout the day or whether ‘tunable-white’ technology should be applied – and its worth bearing in mind that ‘tunable-white’ may have more to do with the emotional factor (see comments above and below),
  • The length of time that this higher level of illuminance is required within the workplace.

Psychology and Emotion: We can run these two factors together because there’s a broader umbrella term that we can use; ‘Aesthetics’. This is something that the lighting designer knows all about because this lies at the heart of everything that we do to create a positive visual environment. There are, however, a few things that we need to learn more about.

The fundamental aim of Biophilic Design is to connect us with elements of nature in our everyday (indoor) lives. A point to consider: introducing living organisms that are not human into the human working environment means we have to learn how to do no harm to other species. Typically, this means special lighting for planting and ornamental horticulture has a huge role to play in maintaining a ‘biophilic environment’.

Plants have their own circadian rhythms and those rhythms can be more sophisticated than our own. Lighting for plants needs to take into account the annual growing cycle as well as what’s happening from day to day. Ornamental plants may be required NOT to flower, so that needs to be factored into light spectra; and plants need to rest at night, so varying illuminance values are important. Research work is proceeding apace on this topic.

Living Wall by Biotecture at Heathrow Terminal 3

Recent research suggests that both plants and their human co-workers work better under a ‘daylight-equivalent’ spectrum. The ‘Sunlike’ range of LEDs from Seoul Semiconductors may provide a guide as to the direction that we need to pursue. It may not be the entire answer, but it suggests a starting point.

It may be in this ‘aesthetic’ aspect of Wellness lighting design that tunable-white lighting comes into its own. There is no evidence that a warmer white light later in the working day has any physiological effect on the body, but there may be emotional benefits to be gained by having this kind of ambient shift in the workspace.

Personal lighting also needs to be addressed, especially in the regard of light colour. Within the overall lighting pattern for the workspace – and regardless of what that pattern might be – there is still an important need for control of lighting by a discrete group (localised) or an individual (task). This may not always be easy to achieve but it should be an ambition of any workspace lighting brief.

Evidence-based design

As we move away from the hard-and-fast performance criteria provided by Lighting Codes, it becomes beholden on the designer to create a detailed design brief for each individual project. The process is called ‘Evidence-Based Design’ (EBD) and it requires bringing together all interested parties and relevant information on working practices / aims and objectives before a lighting performance specification can be identified.

EBD also comes into play at the end of the project and can continue for years into the life of an installation. In a better-organised world, occupants of a space where WELLNESS standards have been applied would be subject to (unobtrusive) continuous physiological and psychological oversight, with results being collated to arrive at a dataset that describes the effectiveness of the building, rather than relying on the random surveys currently being undertaken when funds permit.

We deserve better than this – because this is about the health and welfare of all of us.

Notes on standard recommendations for ‘circadian lighting’

The table below summarises the recommendations given by the WELL Building Standard, DIN SPEC 67600 (in conjunction with DIN SPEC 5031-100) and the Lighting Research Center (LRC) guidelines. Recommendations are highlighted in yellow; the other metrics in the table are worked out based on those recommendations. The only agreement appears to be between the 200 EML recommended by WELL v1 (for the combination of daylight and electric lighting) and the CS of 0.3 recommended by LRC. The most stringent is clearly DIN SPEC 67600, whilst the least stringent is, arguably, the WELL Standard.

As well as considering the efficacy of LEDs, we also need to look at the make-up of the light spectrum produced. At least one lighting company is offering a tweaked light spectrum for its LEDs. The BIOS SkyBlue LED shifts the basic blue ‘pump’ from the ‘conventional’ 430nm peak up to 490nm in order to match the melanopic response at 490nm.  These LEDs are only available in 3000K – 3500K – 4000K versions (and have a colour performance of >80 CRI + R9:90+, which might upset, or confuse, a few specifiers). But this might suggest that less overall illumination is needed to achieve a level of EML. But it still doesn’t address the question of length of exposure.

Notes to the table above:

1 Photopic illuminance (visual lux) is calculated for CIE standard illuminant D65 (daylight 6500K). This metric varies with spectral power distribution for other light sources. For D65, MDEI and photopic illuminance are equal.

2 WELL v1 recommends at least 200 EML (from both daylight and electric lighting) for at least the hours between 9:00 AM and 1:00 PM for every day of the year.

3 WELL v1 recommends at least 150 EML from electric lighting alone. This is also maintained in WELL v2 for 1 point; at least 240 EML are recommended in WELL v2 for 3 points. If sufficient daylight is available (separate criteria are given), the two targets for electric lighting in WELL v2 become 120 EML for 1 point and 180 EML for 3 points.

4 DIN SPEC 67600 recommends a vertical illuminance of 250 lux at 8000K at the eye, for several hours preferably in the morning. DIN SPEC 5031-100 gives the method for translating this into MDEI as well as into photopic illuminance for any spectrum. DIN SPEC 67600 also gives an example of variable lighting settings for offices: 250 lx and 8000K (or 239 lux MDEI) at the eye between 8am-10am and 1pm-2pm; 200 lx and 3000K at the eye (or 99 lux MDEI) between 12pm-1pm and 6pm-8pm; illuminance corresponding to the requirements of the visual task for the rest of the day/night.

5 “For circadian entrainment, a lighting system should deliver a CS ≥ 0.3 throughout the day, or at a minimum provide a CS ≥ 0.3 during the first 2-3 hours of the morning. For alertness at other times of day, the system can continue to deliver a CS ≥ 0.3 throughout the day or use long-wavelength (red) light as supplemental lighting during the afternoon.” (Source:

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