LIGHTING
alertness, refresh attention, and carry out a host of vital biological functions. As the most powerful external cue for circadian entrainment, the attributes of tunable white artificial daylight are now benchmarked. Most dynamic lighting systems can affect the human circadian system. The impact of those systems can be quantified in terms of two prevalent metrics. On the one hand, BREEAM, in partnership with The WELL Building Standard, has opted for the Equivalent Melanopic Lux (EML), whereas in the US, the agency responsible for all federal buildings opted to use the Circadian Stimulus (CS), the metric developed at the Lighting Research Center (Rensselaer Polytechnic Institute), which is now the Light and Health Research Center at Mount Sinai.
Foster + Partners’ NASA Project IRIS in 3D Prototype Outfitter Demonstrator.
both our metabolism and our perception of time, whereas compressed spaces raise both our cortisol levels and speed up our perceived sense of time (lack of visual exit points generates stress over time).
Illusory overhead sky view In 2015, a second study by the Texas Tech University team, carried out in collaboration with Covenant Health Hospital, in Lubbock, Texas, set out to study the effect of an illusory overhead sky view in a patient room that (interestingly enough) also featured a real window view. However, the window view in these patient recovery rooms did not overlook a green space or a water feature, but an adjacent building. Studies have found that buildings and materials like glass, steel, and concrete lack the restorative benefits of biophilic materials like stone, wood, or brick. A view to an urban landscape is no substitute for a view to a natural landscape. The paper, ‘The Impact of Simulated Nature on
Patient Outcomes: A Study of Photographic Open Sky Compositions’, was published in the peer-reviewed HERD Journal.7
The study examined the behavioural and clinical
IRIS Revelation render, Solothurn Hospital, Switzerland.
impact of overhead sky imagery specifically captured, composed, and installed, above the ceiling plane to generate the convincing illusion of vertical volume (spatial cognition). The eight-month study analysed data from 181 patients who were assigned to identical spaces where the only environmental variable separating the experimental from the control rooms was a 1.2 m x 1.8 m. Luminous SkyCeiling (virtual skylight). The single-blind study found a
difference in acute stress of over half (53.40%), and a difference in anxiety of over a third (34.79%) in patients assigned to the experimental rooms where the illusory skies had been installed.7 At the time, Dr. Pati, the lead investigator, noted that his team was the first to study and quantify a simulated overhead view to nature designed to be perceived not as a representational or decorative scene, but as a bone fide perceived overhead opening above the patient bed. Today, leading tunable lighting
systems aim to support our sleep/ wake cycle and performance by providing the appropriate levels of light and spectrum that our physiology requires to maintain
64 Health Estate Journal August 2025
Differences in metrics It is worth noting that the EML metric is based on the spectral sensitivity of the intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) that contain a photopigment – melanopsin, a light-sensitive protein. However, it does not account for the absolute sensitivity of the circadian system – from threshold to saturation – as conventionally measured by physiological outcomes like melatonin suppression (the hormone that regulates sleep) or circadian phase shifting. In other words, EML alone cannot be used to predict a biological response. Similarly, illuminance levels (measured in lux) alone cannot predict visual performance. More information is needed in both cases to predict a biological response. In contrast, the Circadian Stimulus (CS) metric incorporates both the absolute and the spectral sensitivity of the circadian system. The CS model is grounded in the neurophysiology and neuroanatomy of the retina, and represents how light signals received by all photoreceptors – ipRGCs, rods, and cones – are integrated to stimulate the master biological clock (the Suprachiasmatic Nucleus). It is important to emphasise that the CS model is a mathematical construct that approximates the circadian phototransduction mechanisms in the retina; that is, the process by which light is converted into neural signals for the regulation of biological rhythms.
In order to predict nocturnal melatonin suppression or
phase shifting in an interior space, additional information about timing, duration, and spatial distribution of the luminous stimuli is needed. With that in mind, CS can be used as a validated light stimulus metric for predicting the effectiveness of a light source in suppressing melatonin and shifting circadian timing. While these outcomes are often studied in night-time contexts, the same CS framework applies to daytime lighting design, where circadian stimulation through melatonin suppression is a desired outcome for supporting alertness and health. Using manufacturer data and spectrum-based software
tools like Dialux or Agi32, EML, based on the CIE S 026/E:2018, is fairly easy to calculate for architects and lighting designers.8
The foundations of the CS metric are
more comprehensive, but more complex to calculate. This is why the LHRC offers a CS calculator (https://cscalc.
light-health.org/) that facilitates converting photopic illuminance and spectrum to CS, which a knowledgeable lighting designer can work with. On a related note, a key aspect that has got lost in
the mix is that daylight has two other equally restorative effects on our psycho-physiology aside from circadian entrainment. Daylight also modulates focused attention through its intensity variability and its volumetric or spatial nature. In evolutionary terms, our physiology has mapped daylight under the sky’s vast blue canopy. That’s the reason why the ipRGCs are also called the body’s blue-sky detectors. These specialised cells respond to light from above the line of sight.
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