Analysis of circadian stimulus allowed by daylighting in hospital rooms

1 Analysisofcircadianstimulusallowedbydaylightinginhospitalrooms *Ignacio Acosta, Instituto Universitario de Arquitectura y Ciencias de la Construcción, Universidad de Sevilla Russell Leslie, Lighting Research Center, Rensselaer Polytechnic Institute, Troy, New York, USA Mariana G. Figueiro, Lighting Research Center, Rensselaer Polytechnic Institute, Troy, New York, USA Corresponding author: Ignacio Acosta Instituto Universitario de Arquitectura y Ciencias de la Construcción Universidad de Sevilla, 41012 Seville, Spain Tel.: 0034647550654 Email: [email protected] Abstract Light is the major synchronizer of circadian rhythms to the 24-hour solar day. Compared to the visual system, the circadian system requires more light to be activated and is more sensitive to short-wavelength light. Without access to daylight, or electric lighting providing comparable amount, spectrum, distribution, duration, and timing, human health and wellbeing may be compromised. This may be particularly true for those confined indoors, such as patients in hospitals and residents in eldercare facilities. Architectural and design features, including window size, surface reflectances and furniture placement, impact circadian stimulus levels. The present paper details results of simulations used to determine percentage of days that patients would receive a minimum level of circadian stimulation as a function of different window-to-façade ratios, surface reflectances, and latitudes. Keywords: circadian stimulus, daylighting, window, lighting simulation, hospital 1.Background Window design is the key element for allowing daylight inside buildings1 and a proper design can improve thermal comfort and save electric lighting energy.2 Research efforts have investigated how windows provide daylight in a space in terms of the light distribution and potential lighting energy savings.3, 4 Other studies evaluated shading devices,5, 6 defining the effect of blinds or overhangs on daylight availability inside buildings, as well as the relationship of these shading devices with electric lighting.7 Recent studies have also examined the relationship between daylighting and health outcomes.8, 9 In addition to enabling us to see, light reaching the retina has a profound effect on human health and wellbeing via its impact on our circadian rhythms that regulate sleep, mood, and alertness. Circadian disruption is associated with long-term health risks, including diabetes, obesity, cardiovascular disease, and cancer.10-12 Light is the major synchronizer of circadian rhythms to the Earth’s 24-hour light-dark cycle. Circadian rhythms in humans, such as the sleep-wake cycle, repeat approximately every 24 hours. In the absence of any external cue, human circadian rhythms run with an average period slightly greater than 24 2 hours (approximately 24.2 hours). Morning light resets the biological clock daily and promotes entrainment to local time on Earth. Lighting characteristics affecting vision differ from those affecting the circadian system. The circadian system, as measured by acute melatonin suppression, is maximally sensitive to short-wavelength (blue) light, with a peak spectral sensitivity at around 460 nanometers (nm), while the visual system, as measured by visual acuity, is most sensitive to the middle-wavelength portion of the visible spectrum, peaking at around 555 nm.13-15 The circadian system is dependent on the timing of light exposure. Light that is applied before the minimum core body temperature, which is reached approximately 1.5 to 2.5 hours before one naturally awakens, will delay the clock, resulting in later bedtimes and wake times the following day. Light applied after minimum core body temperature is reached will advance the clock, resulting in earlier bedtimes and wake times the following day.16 While the visual system responds to a light stimulus very quickly (in milliseconds), the duration of light exposure needed to affect the circadian system can take several minutes. It is also important to note that the short-term history of light exposure affects the sensitivity of the circadian system to light; the higher the exposure to light during the day, the lower the sensitivity of the circadian system to light, as measured by nocturnal melatonin suppression and phase shifting.17 Daylight is arguably the ideal light source for synchronizing the human circadian system, providing the right amount, spectrum, distribution, duration, and timing needed for entrainment to local time. Indeed, for millennia daylight was the only light source used by terrestrial species for circadian entrainment. In a modern, 24-hour society in which people spend most of their time indoors, it can be assumed that electric lighting, operated both day and night, blurs the distinction between the two, thus compromising circadian entrainment. Without access to daylight, or electric lighting providing comparable amount, spectrum, distribution, duration, and timing, human health and wellbeing may be compromised. This may be particularly true for those confined indoors, such as patients in hospitals and residents in eldercare facilities. Studies to date have examined the effects of daylight on patients18, 19 and their outcomes,20, 21 but have not specifically investigated the effects of daylight on the circadian system of those confined indoors. There are several metrics related to the measurement of daylight through windows. Daylight factor is the simplest and most common metric used to quantify daylight in a space. It expresses the potential illuminance inside a room for the worst possible scenario: overcast sky conditions when there is less exterior daylight.22 Many of the studies of daylight through windows are based on this metric.23, 24 In addition, there are currently new dynamic metrics based on weather data, which require complex calculations through lighting simulation programs.25 Several studies of daylighting have been developed using the dynamic metrics of daylight autonomy or useful daylight illuminance.4 These new metrics complement the daylight factor values, to describe the daylight inside a room.26 Neither daylight factor nor daylight autonomy fully describe the impact of daylight on health and wellbeing, because these metrics quantify light only for the visual system. However, Rea and colleagues have proposed a mathematical model of human circadian phototransduction,15, 27 based on current knowledge of the neuroanatomy and neurophysiology of the human retina, that can be used to quantify one of the many non-visual human responses to light. The model utilizes the empirical, light-induced nocturnal suppression data from published data to 3 characterize the spectral and absolute sensitivities of the human circadian system to light (circadian light; CLA) while still taking into account the neurophysiology and neuroanatomy of the human retina.13-15, 28, 29 From CLA values, it is then possible to determine the magnitude of circadian stimulus (CS). CS is a transformation of CLA into relative units from 0, the threshold for circadian system activation, to 0.7, response saturation, and is directly proportional to nocturnal melatonin suppression after one-hour exposure (0% to 70%) assuming a fixed, 2.3 mm diameter pupil. Figure 1 illustrates the spectral and absolute sensitivities of the phototransduction model. Because CS represents the input-output characteristics of the human circadian system from threshold to saturation, it can potentially be a useful metric to quantify how much daylight from windows is available to those confined indoors.30-32 Based on model predictions, the threshold for activation of the circadian system corresponds to a CS value of 0.1 (see Figure 1). The half saturation constant of acute melatonin suppression, which represents the amount of light needed to achieve 50% of the total suppression amount (i.e., 70%), corresponds to a CS value of 0.35. While there have been no studies linking 35% melatonin suppression to better circadian entrainment, for the purpose of this manuscript, it is hypothesized that exposure to a CS value of 0.35 for at least one hour in the morning would be sufficient to promote daily entrainment. Figure 1. Left: The spectral sensitivity of the human circadian system for narrowband and for polychromatic lights.15, 27 Right: The relationship between the spectrally weighted levels of circadian light (CLA) and the measured levels of nocturnal melatonin suppression for different amounts of narrowband light stimuli used in various published studies. 13, 14, 29, 31, 33 Figure from Rea et al., 2014.34 The present paper details results of simulations used to determine percentage of days that individuals lying in bed or sitting upright would receive a CS value equal to or greater than 0.35 for at least one hour in the morning, as a function of different window-to-façade ratios, surface reflectances, and latitudes. The goal was to illustrate how the CS metric could be utilized to assist with the selection of window characteristics that are likely to promote circadian entrainment and thus benefit health and wellbeing in those confined indoors. 4 2.Methods 2.1.Selectingtheroommodel 2.1.1.Characteristicsoftheroommodel A virtual room measuring 3.0 meters (m) wide × 6.0 m deep × 3.0 m high, the size of a typical hospital room in the studied locations, was used to analyze various daylighting strategies. The room ceiling, walls and floor had a thickness of 0.25 m. Square windows of variable sizes were centered in the 3.0 m wide façade. The double pane window was 0.05 m thick with a visible transmission of 0.75. Daylight simulations were conducted using the window sizes listed in Table 1 with two room surface average reflectances, also listed in Table 1. The inner surfaces of the room were assumed to display Lambertian reflectances. The luminous intensity of reflected light was therefore directly proportional to the cosine of the angle between the observer's line of sight and the surface normal. This study sample did not cover all possible room configurations, but aimed to show a typical room as a case study. Therefore, a total of 12 calculation models were established, as described in Table 1. Table 1. Calculation models (B = bright/high-reflectance surfaces; D = dark/low-reflectance surfaces) Model Window/ façade Window surface (m²) Window dimensions (m) Glass surface (m²) Glass factor Walls reflectance Floor reflectance Ceiling reflectance 10.B 10% 0.90 0.95·0.95 0.64 0.75 0.8 0.6 0.8 10.D 10% 0.90 0.95·0.95 0.64 0.75 0.4 0.2 0.6 20.B 20% 1.80 1.34·1.34 1.41 0.75 0.8 0.6 0.8 20.D 20% 1.80 1.34·1.34 1.41 0.75 0.4 0.2 0.6 30.B 30% 2.70 1.64·1.64 2.22 0.75 0.8 0.6 0.8 30.D 30% 2.70 1.64·1.64 2.22 0.75 0.4 0.2 0.6 40.B 40% 3.60 1.90·1.90 3.06 0.75 0.8 0.6 0.8 40.D 40% 3.60 1.90·1.90 3.06 0.75 0.4 0.2 0.6 60.B 60% 5.40 2.32·2.32 4.71 0.75 0.8 0.6 0.8 60.D 60% 5.40 2.32·2.32 4.71 0.75 0.4 0.2 0.6 80.B 80% 7.20 2.68·2.68 6.40 0.75 0.8 0.6 0.8 80.D 80% 7.20 2.68·2.68 6.40 0.75 0.4 0.2 0.6 2.1.2.Studyonthehorizontalplane In order to estimate CS received at the eye level of a patient lying in bed, CS was calculated at points on a horizontal plane 0.60 m above the floor, a typical height for a hospital bed. To account for different bed locations in the room, the calculation points were spaced at 0.30 m intervals from the window/façade, 0.50 m from the side walls, and 1.00 m from the center row, defining one center axis and two side axes. Figure 2 shows the location of these study points, as well as all variables of the calculation model. 5 Figure 2. Room model for a patient lying in bed. 2.1.3.Studyontheverticalplane In order to estimate CS received at the eye level of a patient sitting upright and facing forward, CS was calculated at points on a vertical plane. The points were located at a height of 1.00 m above the floor, according to the typical height of a chair, 0.50 m from the side wall, and spaced 0.30 m apart as shown in Figure 3. Figure 3. Room model for a patient sitting upright. 6 2.2.Selectingthecalculationmetrics 2.2.1.Calculationofaverageilluminance The average illuminance value was calculated at the study points shown in Figures 2 and 3 using the lighting simulation program DAYSIM 3.1.35 These illuminance values were then used to determine CS at the study points. 2.2.2.CalculationofaverageCS In order to determine the CS values, Rea et al.’s model of human circadian phototransduction was used to estimate CLA from the source spectral power distribution (SPD) and the illuminance levels obtained at each study point.15, 27, 28 The spectrum of daylight was defined by the CIE Standard Illuminant D65,36, 37 roughly corresponding to a midday sun in Western and Northern Europe. CS magnitudes were determined from the calculated CLA values using the following formula: 0.7∗󰇭1 1 1󰇡 .󰇢.󰇮 CS is directly proportional to the predicted levels of light-induced nocturnal melatonin suppression from threshold to saturation, assuming a pupil size of 2.3 mm and an exposure duration of one hour. 2.2.3.CalculationofCSautonomy For the present study, CS autonomy was defined as the percentage of days in the year when CS is equal to or greater than 0.35 for at least one hour in the morning.38, 39 It was hypothesized that achieving this half-maximum saturation would be sufficient to promote circadian entrainment. As shown in Figure 1, the half-saturation constant of acute melatonin suppression, which represents the amount of light needed to achieve 50% of the total suppression amount (i.e., 70%), corresponds to a CS value of 0.35. In a controlled laboratory condition, Zeitzer et al.40 showed that, in response to three consecutive days of a five-hour light exposure (4100 K light source), the human circadian system achieves a half-maximum phase shifting response at light levels ranging from 50 to 160 lux at the eye, which is equivalent to a CS value of 0.04 to 0.15. Given that these results were obtained under controlled laboratory conditions and that the light pulse was given for five hours, we hypothesized that a CS of 0.35 for at least one hour in the morning should be sufficient to promote circadian entrainment. Using a CS calculator based upon the model of phototransduction15, 27 it was determined that the average illuminance of CIE Standard Illuminant D65 needed to meet a CS of 0.35 corresponds to 233 photopic lux at the eye. CS autonomy was thus determined by calculating daylight autonomy for this photopic illuminance value between 8:00 AM and 12:00 PM. Since the human circadian system has, on average, a period slightly greater than 24 hours, morning light is needed to advance the biological clock and promote circadian entrainment to local time on Earth; therefore, the CS autonomy metric focuses on providing the patient with morning light. It is important to keep in mind that, if possible, all-day high circadian stimulation would be preferred, but given other constraints, we 7 are proposing that, at a minimum, a CS of 0.35 should be provided during the morning hours. The analysis of the results determines the percentage of days throughout the year that the CS value is equal to or greater than 0.35 for at least one hour in the morning. 2.2.4.Calculationofmaximumdaylightautonomy In order to have an illuminance-based glare analysis, maximum daylight autonomy was defined, for the present study, as the percentage of the year when the illuminance reached a high value due to the incidence of sunlight, effectively a “glare zone” where sunlight falls inside the room. The maximum daylight autonomy was calculated for five-minute intervals throughout the year. The glare from direct sun may be too high for patient comfort, although it can be beneficial for their health and wellbeing.41 The maximum daylight autonomy was calculated by DAYSIM 3.1 using the daylight autonomy calculation with a threshold of 5000 lux. 2.3.Selectingthecalculationconditions 2.3.1.Locationoftheroom Two locations in Europe at different latitudes and with different sky conditions were chosen to illustrate the impact of variation of daylight availability: (1) London, UK, at 50° north latitude and with predominantly overcast skies; and (2) Madrid, Spain, at 40° north latitude and with predominantly clear skies. Weather data for both locations are defined by the EnergyPlus reference.42 2.3.2.Orientationofthewindow The window orientation was north for all calculation models and both locations. Illuminance values are lower for north-facing windows,4 therefore representing the worst case scenario for daylight illuminance and CS. 2.4.Selectingthecalculationprogram DAYSIM 3.1 is a validated RADIANCE-based daylighting analysis tool that uses a daylight coefficient approach combined with the Perez all-weather sky model43 to predict the amount of daylight in and around buildings, based on direct normal and diffuse horizontal irradiances taken from a climate file. DAYSIM was developed to provide a more efficient calculation of illuminance or luminance time series under varying sky conditions than that provided by RADIANCE in its original form. This lighting software has been validated by several researchers.35, 44 The calculation parameters used by this program in this research are shown in Table 2. Table 2. Parameters of the calculation program Ambient Bounces 7 Ambient Divisions 1500 Ambient Super-Samples 100 Ambient Resolution 300 Ambient Accuracy 0.05 Limit Reflection 10 Specular Threshold 0.0000 8 Specular Jitter 1.0000 Limit Weight 0.0040 Direct Jitter 0.0000 Direct Sampling 0.2000 Direct Relays 2 Direct Pretest Density 512 3.Results 3.1.AverageCS Following the methodology described above, Figure 4 shows the average CS values at the study points representing the eyes of a patient located in London, lying face up on a bed in a room with high reflectances and a north-facing façade. CS values are shown for different window sizes ranging from 10 to 80% of the façade area. The maximum daylight autonomy (i.e., areas likely to have direct sun) is also indicated, measured at the study points on the central axis. Figure 4 also shows the average CS values at the study points located on the vertical plane, representing the position of a patient sitting upright and facing forward. As expected, the CS values are higher in the zone near the façade, gradually decreasing toward the back of the room. Obviously, the larger windows provide higher CS. However, as can be deduced from the results, the window area and resulting CS values are not directly proportional. Rooms with window-to-façade ratios of 60 and 80% provide similar CS values; therefore, there is no significant benefit to having a particularly large window. The CS values observed on the side axes are similar to those measured on the central axis, except in the rooms with small windows (window-to-façade ratios of 10-30%). That is to say, windows with an area higher than 40% of the façade allow an evenly distributed CS. Significantly, daylight provided greater CS on the vertical plane compared to the horizontal plane, suggesting an advantage for patients to be sitting rather than lying down for a period in the morning. For the room with high reflectance walls in London, a window with an area equal to 20% of the façade provides a CS value higher than 0.35 only within the first 3.0 m from the façade for a patient lying in bed. However, for a sitting patient, this distance increases to over 5.0 m. To provide the CS criterion in the entire room, a window-to-façade ratio of at least 40% is needed for a patient lying down and 30% for the patient sitting up. For the room with low reflectance surfaces in London, a typically overcast location, even a window with an area equal to 80% of the façade is not large enough to provide a CS value of 0.35 in the entire room for a lying patient; however, a window-to-façade ratio of 40% or higher meets the criterion CS for a sitting patient, even in this low reflectance room. 9 Figure 4. Average circadian stimulus (CS) values calculated between 8:00 AM and 12:00 PM on horizontal and vertical planes for a high reflectance room in London. The maximum daylight autonomy (DA) is also indicated, measured at the study points on the central axis.