Volume 7 Issue 1 > pp. 13-24 • doi: 10.15627/jd.2020.2

# Application of Different Circadian Lighting Metrics in a Health Residence

Nicola Busatto,* Tiziano Dalla Mora, Fabio Peron, Piercarlo Romagnoni

Author affiliations

Department of Architecture and Arts, IUAV University of Venice, Dorsoduro 2206, 30123 Venezia, Italy

* Corresponding author. Tel. +39 041.257.1234
nbusatto@iuav.it (N. Busatto)
tdallamora@iuav.it (T. Dalla Mora)
fperon@iuav.it (F. Peron)
pierca@iuav.it (P. Romagnoni)

History: Received 21 November 2019 | Revised 13 January 2020 | Accepted 20 January 2020 | Published online 20 February 2020

Citation: Nicola Busatto, Tiziano Dalla Mora, Fabio Peron, Piercarlo Romagnoni, Application of Different Circadian Lighting Metrics in a Health Residence, Journal of Daylighting 7 (2020) 13-24. http://dx.doi.org/10.15627/jd.2020.2

Figures and tables

## Abstract

During the last three decades, various scientific researchers analysed the connection between natural elements and human life, both as far as well-being and productivity. This study simulates circadian lighting metrics on elderly occupants in a healthcare residence, with different types of metrics. It presents an application of some metrics that try to quantify the effects of natural light on the human being. Circadian Action Factor, Circadian Stimulus, and Equivalent Melanopic Lux are considered. Starting from the spectral power distribution parameter, the three metrics are compared. The final aim is to build up a standard technique to represent the effect of key design choices on the accomplishment of endorsed circadian framework boost for healthcare residences occupied by elderly people.

## 1. Introduction

The main objective of the paper is to demonstrate how the three circadian assessment metrics acv, CS, EML can be used and compared in simulating the effects of light within the built environments, through the verification of the illuminance on the vertical plane and the SPD of light sources. One more objective is the implementation of the circadian effects of light sources in relation to the activities carried out by patients in the assisted healthcare residence. The novelty of this study consists in the use and subsequent comparison of all the acv, CS, EML metrics in the currently literature regarding the evaluation of the effects of light from the circadian point of view in the built environments. The aim is to demonstrate how the light and the activity carried out in the environments are closely related to human comfort and that therefore the metrics also give design indications. The expected results concern the validity of the results of acv metric, currently outdated in comparison to the other more updated ones, and the demonstration that circadian metrics give clear and coherent indications to improve the light in relation to the people activities.

## 2. Method

The paper methodology is structured as follows: in the first part different metrics for calculating the circadian rhythm are considered according the current state of research and literature; in the second part a case study of a healthcare residence is presented to evaluate the circadian effects in two rooms, testing the real state and proposing an improvement scenario.

The calculation involves measurement and simulation phases. In fact, the purpose of the measurement phase is to obtain spectral radiance values for each color within the case study. For the evaluation of the spectral radiance the measurements are carried out with a spectroradiometer. The instrument used is the Spectrascan PR-650 spectrometer radio from Photoresearch. The measurement points, locations and input, output data are specified in the following paragraphs.

As regards the evaluation of the obtained results, this study presents only the values referred to the vertical plane of the point of view since the most relevant for the evaluation of the circadian stimulus. In fact, for the evaluation of the light effects by circadian point of view, the simulations must be carried out on the vertical plane corresponding to the eye of the observer in order to evaluate how many lux of illuminance reach the retina: the photoreceptors of the circadian system, namely the ganglion Melanospina cells, are properly placed inside the retina and are mostly stimulated by light from above. So, the evaluation of the light from the circadian point of view is different from the evaluation of the lighting only from the visual point which occurs only on the horizontal plane. This consideration is deeply described in the study of Brainard [5] and Thapan [6] about the melatonin suppression by different quantities of lux measured directly inside human eyes and therefore tested on vertical plane. At last a discussion of the main outcomes is presented.

The general photometry based on visual performance of light is not sufficient to explain light by a circadian perspective. Gall [20] proposes a circadian action function C (λ), obtained directly from experimental studies by Brainard [5] and Thapan [6]: it concerns on the melatonin suppression actuated by various wavelengths of the light stimulus, using an immediate strategy for relating the circadian sizes with those photometric by means of the circadian action factor acv. The phenomenon of spectral opposition raises doubt about the circadian photometric proposed by Gall and over the most recent couple of years. Figueiro et al.  has proposed a metric called circadian stimulus (CS) that has assumed a key role in the field of lighting design for healthy buildings [23]. A third approach refers to melanopic illumination calculates suppression of melatonin relying on the composition spectral value of the light weighted irradiance to cornea with respect to the one of a kind spectral contents of the 5 types of receptors.  The researchers recorded the light exposures of each of the photoreceptors in the greater whole form is a power distribution spectral corneal, through the calculation of irradiance [24]. Despite the huge range of research carried out, it was not viable to reach the description of light as a one-dimensional quantity which predicts non photographic optical responses. To quantify the effects of the 5 types of α-opic equivalent daylight illuminance (lx), Al Enezi [25] and Lucas [1], proposed a new metrics and introduced the α-opic illuminance concept Eα. The characteristics of the different metrics are presented in the following chapters.

### 3.1. Circadian action factor acv

Utilizing the action factor, acv, Gall [20] set up a circadian photometry, and the values can be obtained from radiometric quantities through the circadian affectivity C (λ) rather than the photopic affectivity V (λ). In Fig. 1, it is conceivable to take note of that the circadian activity capacity bend decided experimentally by researchers is maximally sensitive to short wavelengths (blue zone) regarding visual sensitivity curve [5,6].

Figure 1

Fig. 1. Circadian action function C (λ) of [20].

This leads to see the amount of artificial lighting with high correlated colour temperatures (approx. 4,500 K), currently used for lighting the building, is not practicable in suppressing daytime melatonin in contrast to natural light, which is rather rich in short wavelengths (blue and purple).

Gall outlined an immediate strategy for relating the circadian quantities with photometric ones, by the characterization of the following circadian quantities: flux Φc, intensity Ic, illuminance Ec, luminance Lc [20].

$a_{cv}=\frac{Φ_{cρ}}{Φ_c} = \frac{K_m ∫_λΦ_{eλ}C(λ)d(λ)} {K_m ∫_λΦ_{eλ}C(λ)d(λ)}$

where Km represents the maximum value of visual spectral efficiency equal to 683 lm/W, Φel represents the spectral distribution of radiant flux, Φcp is the reflected circadian flux.

The circadian efficiency factor acv describes the connection between circadian quantities (for this situation photometric circadian quantities) and photometric quantities (in the equation denominator Φ signifies luminous flux) [20].

The Circadian action factor acv is currently the means through which you can get circadian quantities from those photometric quantities.

$E_{ec}=\frac{a_{cv}}{K_m}Ev$

where Ev is the Illuminance.

The Circadian action factor acv can be accepted as an indicator for the circadian effect of a light source. Concerning lighting design mindful of circadian perspective, (Table 1) Gall proposes ideal combinations between individual activities, the Circadian activation factor acv and colour temperature of light sources [20]. This is a coefficient that roughly describes the circadian efficiency of the various colours of light, yet it doesn't consider the phenomenon of spectral opposition [26].

Table 1

Table 1. Relationship between people activities, Circadian action factor, and colour temperature.

The significance of the spectral distribution of light emission is connected to the normal for yellow/blue and red/green spectral opposition of the circadian framework: due to this phenomenon, with a similar intensity of the blue component, a source containing a proportional yellow component stimulates significantly less than one that has these two components strongly unbalanced in favor of blue [27]. Figueiro et al. [28] demonstrated that only 18 lux delivered by a blue LED are progressively proficient in the suppression of 450 lux melatonin than white light created by a mercury vapor lamp having a spectrum characterized by a peak in blue and one in yellow. The phenomenon of spectral opposition of the circadian system therefore makes it difficult to set up an immediate connection between the intensity of  luminous stimulus and the level of melatonin suppression without also considering its real spectral composition: in other words, it is not possible to add the contribution of the different lengths wave taken individually. The way that the circadian system is progressively delicate to blue light decreasing the yellow-red components leads seeing the amount of the white artificial light, at present utilized for lighting buildings [27].

Figure 2

Fig. 2. Nocturnal human melatonin suppression data (left ordinate), vs CL quantities (abscissa) predicted by the model of Rea et al. [29].

Table 2

Table 2. Relationship between people activities and Circadian Stimulus (CS).

Lighting Research Center Rensselaer Polytechnic Institute has promoted a CS [26] to help professionals lighting designer to select sources and levels of light, which allow adequate exposure of buildings to light. This tool facilitates the calculations of CLA and CS for different spectra of light sources, to some spectrums of predefined light sources are already set up, but it is also you can add new ones. CLA circadian light is irradiance at the cornea weighted to reflect the spectral sensitivity of the human circadian system as measured by acute melatonin suppression after a one-hour exposure.

CL is spectrally weighted irradiance for the human circadian system, a term more comparable to illuminance, which is spectrally weighted irradiance for the human visual system.

The units of CL (spectrally weighted irradiance in W/m2) are not particularly intuitive to a user, a normalized quantity, CLA, was derived to more easily compare CL values with values of photopic illuminance, in (lx).

During the design plan, the designers must decide the project objectives, that is if they want to reach a level of relaxation or vigilance. Once the basic condition is established, the, which will vary throughout the day in order to promote health and improve the functionality of the occupants [23].

The tool offered by Lighting Research Center to encourage calculations of CLA and CS for a few example light source spectra as well as user-supplied light source spectra. Some considerations need to remember when designing with this tool to calculate CS:

1. Request the SPD about the light sources under consideration yet maintain cautious no longer to matter solely on their CCTs. While light sources which include higher CCTs (5,000-6,500 K) will usually furnish higher CS, so is no longer usually the case. It is possible, due to the fact example, that a 3,500 K furnish choice give much less CS than a 3,000 K source. Moreover, two light sources rated due to the fact the equal CCT might grant entirely one of a variety CS values depending about theirs SPD. The physiological causes because of this variation keep been studied then are described somewhere else [23];
2. Design for vertical (≈ corneal) illuminance (EV) at the eye, and not just horizontal illuminance (EH) on the workplane [23];
3. Choose light sources that provide the best (EH) to (EV) ratio [23].

### 3.3. Equivalent Melanopic Lux

The approach of melanopic illumination calculates suppression of melatonin relying on the composition spectral value of the light weighted irradiance to cornea with respect to the one of a kind spectral contents of the 5 types of receptors. According to contemporary statements there are at least 5 exceptional kinds of receptors in the human eye. The researchers recorded the light exposures of each of the photoreceptors in the greater whole form is a power distribution spectral corneal, through the calculation of irradiance [24]. Despite the huge range of research carried out, it was not viable to reach the description of light as a one-dimensional quantity which predicts non photographic optical responses. To calculate the values of the 5 types of illuminance, Al Enzi introduced the equivalent α-optical illuminance concept Eα, where α indicates the retinal photopigment for a given organism [25], subsequently taken up Lucas, creator of the EML metric [1]. Given a spectrum, the technique approves the obtainment of the "α-opic" equal illumination for every of the five photoreceptors in the eye (three cones, rods and the ipRGC). The precise α-optical equal illumination a photometric volume associated to the spectral distribution irradiance strength Ee, λ (λ). To render the illuminance melanopic, calculated according to the approach by means of Al Enzi, equal to photopic illumination, Lucas proposed [1] a new function. The new approach proposed by Lucas also offers that the melanopic illuminance is measured through a unit of measure of the Melanopic Lux equivalent (EML) (Table 3). The EML is measured vertically at the height of the observer's eyes and to calculate the equal lux melanopic, so it is indispensable to multiply the illuminance value incident on the vertical plane of the observer (Ev) for a coefficient acquired on the weighting of the light source spectrum (R), in accordance to the Eq. (3):

$EML= L × R$

where L is the visual lux, R is the melanopic Ratio.

Table 3

Table 3. Relationship between people activities and Equivalent Melanopic Lux (EML).

For example, incandescent lights furnish 200 lux in a space produce 108 Equivalent Melanopic lux. If daylight is modelled to furnish the identical visible brightness (200 lux), it will additionally furnish 220 Equivalent Melanopic lux.

The use of this metric was adopted inside a rating system that identifies the parameters of well-being for architectural constructions: the international WELL Building Standard, devised via the International WELL Building Institute [30] to asses and certify the level of health and well-being of the customers inside the environments. The benchmarks that the WELL protocol identifies have been used for healthcare environments in terms of natural and artificial light [24].

## 4. Simulation

### 4.1. Case study

The study model is the RELAXXI R.S.A healthcare residence (Fig. 3) for elderly people located in Noale (Venice). The choice is motivated because the building represents a suitable environment to control natural and artificial lighting in the context of the circadian survey. In fact, it is essential to hold a high level of vigilance and attention in the elderly during sunlight hours, by breaking down the melatonin suppression, as well as resting in the night hours and where most of them have more or less pathologies, such as Alzheimer's or senile dementia and which usually exhibit delays in the circadian rhythm and other sleep-related disorders.

Figure 3

Fig. 3. Healthcare residence view.

Simulations have been performed for two opposite areas in terms of attendance and use. Occupancy times and traditional daily timetable have been recorder through surveys and interviews with medical personnel to drive the preference of environments to be analysed: gym and patient room. The gym is used for rehabilitation, physiotherapy activities and motor rehabilitation and stimulation of patient function in defined time slots, in the morning from 9.00 to 10.00, while in the afternoon from 15.30 to 19.30. The patient's room is mainly used for night and afternoon rest and is occupied mainly from 19.45 to 7.30 awakening time and from 13.30 to 14.30 for patients' afternoon rest. According to the guidelines available today, in the gym should be maintained lighting conditions suitable for rehabilitation activities to maintain high levels of vigilance and concentration of patients, while in the room should be maintained lighting conditions that predispose to rest and relaxation of patients. The simulations provide verification of the Circadian action factor, Circadian Stimulus and Equivalent Melanopic Lux in the state of art and then in a project proposal in the simulations with the measures utilized to enhance the circadian action involving it in relation to the supposed uses and the activities of healthcare residence’s people.

### 4.2. Current state - Gym

Gym room (Fig. 4) is 13.50 meters (m) wide by 7.90 m deep by 3 m high (Fig. 5). The gym room was used to analyse acv, CS and EML with natural and electric lighting. The room provides a ceiling and partitions with RAL 9003 (x = 0.3533 – y = 0.3688) colour while flooring had a RAL 4006 (x = 0.4289 – y = 0.2784), which means “RAL number” as paint colour. Horizontal windows of variable sizes are positioned in the East façade. The windows present a visible transmission of 0.75. Simulations with natural lighting are carried out using CIE D65 Overcast Daylight which represents the worst sky condition. Simulations with electric lighting have been carried out using lamps and SPD presented in the room. Adopted lamps is Philips MASTER TLD – colour temperature 3,000 K (Fig. 6). In order to estimate the (acv), (CS) and (EML) obtained at the eyes of a patient, calculations were set on a vertical plane 1.80 m above the floor. To account for specific point of view of patient, the calculation points were primarily based on three different points of view direction into the simulation room (Fig. 5). The illuminance, for the overcast daylight scenario, simulated at eye level was 195 lx, with electric light Illuminance (lx) = 230. The calculation of Illuminance and acv have been calculated with Radiance simulation program. CS have been calculated with “Circadian Stimulus Calculator” developed by Lighting Research Center [23]. EML have been calculated with “Melanopic ratio data sheet” provided by Well Building Standard, EML = L × R [1], where L is equal to 195 lux and R is equal to 1.3262 for CIE D65. Gym presented 3 lines of lamps with 4 lamps in each line with a low colour temperature of 3,000 K (Fig. 6).

Figure 4

Fig. 4. Gym view at current state.

Figure 5

Fig. 5. Graphical drawings for Gym room: (a) section and (b) plan with allocation and placement of points of view for camera for simulation output.

Figure 6

Fig. 6. SPD of the Philips MASTER TLD – 3,000 K for gym room.

The gym presents the state described in Fig. 7: a circadian action factor acv = 0.91, CS = 0.32 and EML = 258 with natural light and acv = 0.59, CS = 0.21 and EML = 86 with electric light only, primarily given through the presence of lamps with a low colour temperature of 3,000 K and SPD of lamps with a peak around 600 nm. In this case the acv, CS and EML results are in line with the activities of the people in this space that require attention for the rehabilitation of patients during the day; on the contrary, it seems rather low with artificial light as regards the possible activities carried out in the hours without the presence of natural light.

Figure 7

Fig. 7. Gym room in current state: (a) overcast sky conditions and (b) electric light.

### 4.3. Improvement state - Gym

In the state of art gym presents a high circadian action factor acv, CS and EML; to get better it even with the only contribution of the electric light, it is proposed a ceiling of blue paint RAL 5012 (x = 0.2493 – y = 0.2873) and lamps with high CCTs of 6,500 K using the same lamps Philips MASTER TLD (Fig. 8). From the point of view of electric light, it would be necessary to raise the values to obtain at least a 30% of CS. The illuminance, for the overcast daylight scenario, simulated at eye level was 195 (lx), with electric light Illuminance (lx) = 251.

Figure 8

Fig. 8. SPD of the Philips MASTER TLD – 6,500 K for gym room.

The gym presented an acv factor, CS and EML already good in its state of art with only daylighting, but to improve the factors even with the only contribution of the electric light present it was proposed a blue-coloured ceiling and the use of lamps with high CCTs of 6,500 K obtaining a high acv index of 0.79, 0.31 CS and EML of 255 (Fig. 9).

Figure 9

Fig. 9. Gym room in improvement state: (a) overcast sky conditions and (b) electric light.

### 4.4. Current state - patient room

Patient room (Fig. 10), 5.50 m wide by 3.80 m deep by 2.70 m high, the patient room is used to analyse acv, CS and EML with natural and electric light. The room provides a ceiling and walls with RAL 9003 (x = 0.3533 – y = 0.3688), while floor had a RAL 7047 (x = 0.3570 – y = 0.3644). Vertical windows 0.90 m width 2.40 m height were placed in the East façade at first floor. The windows present a visible transmission of 0.75. Simulations with natural light are carried out with overcast sky. Simulations with electric light are carried out using lamps and SPD presented in the room. In order to estimate the acv, (CS) and EML obtained at the eyes of people, calculations are set on a vertical plane 0.85 cm above the floor. To account for different point of view of patient the calculation points are based on three different point of view direction into the simulation room (Fig. 11). The illuminance, for the overcast daylight scenario, simulated at eye level was 291(lx), with electric light Illuminance lx = 340. The patient room presents 3 typologies of lamps with a medium colour temperature of 4,000 K, using the same lamps Philips MASTER TLD (Fig. 12).

Figure 10

Fig. 10. Patient room at current state, (a) during daylight and (b) in artificial light conditions.

Figure 11

Fig. 11. Graphical drawings for Patient room: (a) section and (b) plan with allocation and placement of points of view for camera for simulation output.

Figure 12

Fig. 12. SPD of the Philips MASTER TLD – 4,000 K for patient room.

The patient room offers a suitable circadian action factor acv, CS and EML for the duration of daylight hours with natural light acv = 0.81, CS = 0.39, EML = 386 while with electric lighting acv = 0.66, CS = 0.30, EML = 199 with electric light solely above the bed, primarily given by the presence of light sources with an average colour temperature of 4,000 K using the same lamps Philips MASTER TLD (Fig. 13). In this situation, in evening hours the patient room needs a circadian action factor acv, a circadian stimulus and melanopic lux lower to promote relaxation and the sleep, furthermore the use of appropriate colours of the partitions favourable from the psychological point of view.

Figure 13

Fig. 13. Patient room in state of art conditions: (a) overcast sky conditions (b) electric light.

### 4.5. Improvement state – patient room

In the patient room intervening in the partitions with a green tint RAL 6017 (x = 0.3471 – y = 0.4550),  colour neither warm nor cold which does not amplify the heart rate and provide serenity; the use of lamps with CCT of 2,700 K using the same lamps Philips MASTER TLD (Fig. 14), keeps an high acv, CS and EML at some point of the daylight hours, however at the same time is reduced considerably in the evening hours with artificial light only, ideal to promote relax and the sleep of patients. With Overcast daylight was Illuminance (lx) = 291, while with electric light Illuminance (lx) = 190.

Figure 14

Fig. 14. SPD of the Philips MASTER TLD – 4,000 K for patient room.

The patient room showed acv factor, CS and EML already excessive in its state of art with daylighting, but with the solely contribution of the electric lighting acv factor, CS and EML too high all through the evening hours and not in line with the benchmark proposed (Fig. 15). Intervening in the colour of the partitions with a green tint, and the use of lamps with low CCTs of 2,700 K using the same lamps Philips MASTER TLD (Fig. 14), a high acv factor, CS and EML is maintained all through the daytime, however at the same time it lowers significantly in the night-time with electric lighting, ideal to promote relaxation, sleep and the melatonin secretion.

Figure 15

Fig. 15. Patient room in project proposals: (a) overcast sky conditions (b) artificial light.

## 5. Discussion and results analysis

### 5.1. Daylighting

The research gives numerous insights. Simulation of the acv, CS and EML provide evidence of dependence not only on colours of the walls, but additionally on the spectral interreflections inside the environment; great contribution is given by the quantity of illuminance EV at the eye of patient, which is a characteristic of the quantity and dimension of windows in the case of natural light simulation; in this case the CIE D65: Overcast Daylight 6,500 K is very rich in the short wavelengths (blue and violet), then it stimulates the circadian maximum activity in humans.

In this research, the analysis the state of the art (Figs. 16-18) of analysed areas always gives higher acv, CS and EML than benchmark proposed by Gall, Rea and Well Building Standard to promote melatonin suppression. This is an excellent result with regard to daytime with daylighting, as in a health residence for non-self-sufficient elderly people is necessary to hold a high level of vigilance and attention in the elderly patients, breaking down the melatonin suppression, where most of the occupants are affected by serious diseases such as Alzheimer's or senile dementia, which typically exhibit delays in the cycles of circadian rhythm and different sleep disorders.

Figure 16

Fig. 16. acv factor with CIE D65: overcast daylight.

Figure 17

Fig. 17. CS with CIE D65: overcast daylight.

Figure 18

Fig. 18. EML with CIE D65: overcast daylight.

### 5.2. Electric lighting

The electric lighting simulation scenario is specific from the daylighting, as a number of electric light sources with distinct characteristics in terms of colour temperature and spectrums have been tested. Based on the literature research and then through the simulations, it was viable to test as light sources with CCTs equal to 6,500 K, characterised by the presence of blue wave length, make a contribution to increasing the acv [20] and CS [23]. The values of the state of improvement are higher than current state in the gym, because the goal was to improvement circadian efficacy of light in relation of activities. Conversely, the use of lamp with SPD lower than 3,000 K decreases the impact of circadian stimulus. In addition to altering lamp types in the environments where necessary, similarly verification on room colour affect has been performed. Results have shown that colour contributes in the amplify and/or reduce the circadian action factor acv, as the calculation of the distribution of the circadian illumination is affected through inter-reflections flow between the surfaces making up the indoors itself [31], while in the CS approach partitions colours do not affect results (Figs. 19-21).

Figure 19

Fig. 19. acv factor with electric light.

Figure 20

Fig. 20. CS with electric light.

Figure 21

Fig. 21. EML with electric light.

Electric light design goals to reduce the melatonin suppression during the day to promote higher activity of the patients, and the capacity to manage and follow the patient's circadian rhythm in the evening, promoting the secretion of melatonin during the night-time hours. It is relevant to observe that keeping appropriate values of acv, CS and EML offers advantages not solely for the health of the elderly patients and their biological clock adjustment, but additionally in terms of night-time staff management. The electric lighting proposals have been designed to follow the circadian rhythm of patients in accordance to the activities and intended use.

In particular, the important characteristics of the light that influence the factors are quantity of light, direction, time of the stimulus and spectral distribution. In addition to the physiological aspects, also psychophysical reactions of people in connection with unique warm and cold colours of light and environments have been considered, even if in a qualitative way. As considered in the research performed by Figuero [32], particular colours used to facilitate the circadian rhythm normally contrast in reactions of patients from a psychological point of view. In this work, authors tried to balance physiological and psychological well-being trying to use appropriate colours for each psychophysiological field. In fact, the green coloration has no precise psychological impact, but still stimulates serenity, does not amplify blood pressure, increases the optimism and eliminates depressive symptoms.

## 6. Conclusions

This study compares different metrics, Circadian action factor (acv), CS and Melanopic Lux (EML), to simulate the melatonin suppression for human health needs in a healthcare residence. Even if the acv is considered a surpassed method and it doesn’t contemplate the phenomenon of spectral opposition results got by means of the simulations are comparable with the performances given by the CS and EML. In fact, in the simulations of the two study rooms, the results show that ‘is coherence between the values of the metrics analysed results shown the key role of the spectral component of the light (SPD) for the stimulation of circadian system. Actually, in daylight simulations the metric values are high because the spectral component is rich in short (blue) lengths that stimulate the circadian effect. Results of the performed simulations confirmed the relationship between the level of eye lighting on the vertical plane and melatonin suppression in humans being, according to the scientific literature.

Results have shown that colour contributes in the amplify and/or reduce the circadian action factor acv, as the calculation of the distribution of the circadian illumination is affected through inter-reflections flow between the surfaces is demonstrated in the simulation performed in this study and is based on K. Wandachowicz [31].

In fact, simulation output shown how the acv factor is influenced not only by the spectral component of the light but also by the colour of the room surfaces where the inter-reflections interact: for example on gym room the acv factor increases from the current state to the improvement state by only changing the colour of the ceiling with blue. The same effects are verified also for CS and EML methods in terms of change colour, especially due to the change is due to the change in the light source. With reference to the illuminance value, the only change in the colour of the surface does not affect and change the illuminance values for CS and EML metrics.

The results obtained are important because today lighting designers perform simulations to predict only horizontal plane illuminance, where this method only allows vision to be guaranteed. But in lighting design of natural and electrical light, suitable stimulation of the circadian cycle is also influenced by the illumination at eye level on the vertical plane, which deserves dedicated simulation work. Modern society lifestyle is based on people studying, working and living, in confined spaces where the use of electric light is the basic lighting. Learning and investigating the influence of lighting and its effects in humans being result very important for a sustainable development in regard to human health. This is important in reference to the elderly people of health and hospital environments, where it will be indispensable to get a suitable and innovative spaces from a lighting point of view in order to allow the best performance of visual comfort, according to the psychophysiological point of view.

Future developments include tests and simulations on the same metrics considering the combination of daylight and electric light, and in addition also the effects of a clear sky and intermediate sky in order to obtain values comparable with the real situation of use of the environments.

## Acknowledgement

The authors would like to acknowledge to all residents, and staff of the RELAXXI R.S.A. Santa Maria dei Battuti for their cooperation in this research.

## Contributions

The authors have contributed equally.

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