Modeling dynamic aspects of human nonvisual responses to light

Since 2002, when the first reports on the discovery of a novel type of mammalian ocular photoreceptor were published, a new field of study at the intersection of photobiology and architecture started to emerge. These novel non-rod, non-cone photoreceptors in the retinal ganglion cell layer are the primary mediators of nonvisual responses to light in humans, including synchronizing circadian rhythms and directly alerting the brain. This study aims to understand how these nonvisual responses evolve over time with respect to changes in the intensity, spectral composition and exposure duration of light stimuli. The ultimate goal is to incorporate these effects of light into building design. Recent studies in the field of photobiology provide us with information about the human nonvisual responses to light. Researchers have identified intensity, spectrum, duration, history and timing of light exposure as important parameters that influence the responsiveness of the nonvisual system. Experimental research quickly reaches its limitations, because it is infeasible to carry out a complete experiment with respect to all parameter combinations. Mathematical models are the method of choice to enable such analysis. A block-structured model is proposed that combines linear filters, a nonlinear term and a feedback mechanism. The linear filters reflect the temporal processing between the light stimulus and the output response. The nonlinear term is the sigmoid-shaped intensity-response curve that receives input from the feedback mechanism to regulate the system’s response with changes in prior light history and timing of circadian phase. Moreover, the spectral sensitivity of the nonvisual system is modeled as a time-varying function. Based on this model, which takes the intensity, spectrum and duration of light exposure into account, it is possible to compare the nonvisual efficiency of different spectra as a function of duration. Results demonstrate that duration is an important parameter in addition to spectral sensitivity when comparing nonvisual effects of different light sources at low light intensities. This model provides a framework that can inform designers about the effects of lighting on human health and wellbeing in real-life settings. The ultimate goal of this work is not to reveal the underlying functionality of the retina, but rather to predict human nonvisual responses to light using mathematical models and test the predictions experimentally. Modeling dynamic aspects of nonvisual responses to light is important to understand the dynamic relationship between light and human nonvisual responses that occurs in real-world settings.

Healthy lighting: integrating non-visual responses to light into building simulation

María Lovísa Ámundadóttir

The goal of this thesis is to develop a computer-based lighting simulation framework capable of predicting human non-visual responses to light and to validate novel guidelines that can inform designers about how lighting might affect human non-visual responses in the built environment. The framework consists of four steps, including a mathematical model that accounts for spectral and dynamic variations in light exposure. As a part of the framework, different light exposure patterns are generated to simulate the effects of occupants’ actions, movements, and activities. The results are interpreted based on pre-defined design goals. Light is one of the most significant factors that affect human health and wellbeing in the built environment. Since 2002, when the first reports on the discovery of a novel type of photoreceptor were published, a new field of study started to emerge at the intersection of photobiology and architecture. The novel photoreceptors are the primary mediators of non-visual responses to light in humans, including synchronizing circadian rhythms and directly alerting the brain. These new findings emanating from the photobiology field have sparked a growing interest in the role of lighting design on human health and wellbeing. The novel photoreceptors contain the photopigment meanopsin. Melanopsin is more sensitive to short- wavelength light, with a peak sensitivity that is blue-shifted (λmax ≅ 480 nm), relative to the photopic visual system (λmax ≅ 555 nm), which is dominated by the response of cone photoreceptors. In addition to the difference between the photoreceptors’ spectral sensitivity, researchers have identified intensity, duration/pattern, history, and timing of light exposure as important variables that control the non-visual light response in humans. Considering the blue-shifted sensitivity of the melanopsin-containing photoreceptors, the current recommendations for lighting, which are based mainly on visual criteria, may not provide the necessary amount of light to synchronize important physiological and behavioral rhythms to the 24-hour day. Moreover, a number of recent studies have demonstrated that exposure to bright light can improve alertness, performance, and mood. However, exposures to light at night can disturb circadian rhythms, such as hormone production and sleep-wake cycles, and have been linked to increased risk of cancer. Lighting simulation software tools are designed to evaluate visual requirements and comfort taking into account the stochastic fluctuations linked to the climate and to the behavior of the buildings’ occupants. The evaluation compares simulated light intensity values on a horizontal plane with static threshold values. Since the non-visual system responds slowly to light exposure and adapts to changes in light intensity and spectral composition over much longer time periods than the visual system, the lighting simulation software tools cannot be applied directly to evaluate the non-visual response to light. The response must be evaluated based on dynamic threshold values, which depend on intensity, spectrum, duration, history, and timing of light exposure. Currently, there is no mathematical model that incorporates all five variables to predict the non-visual effects of light on humans. Further, the movements of humans must be simulated to account for the amount of light received at the eye. Thus horizontal sensors on a plane must be replaced with vertical sensors at the eye level that can rotate and move around. The importance of building simulations is growing with increasing complexity of building design and higher performance requirements regarding sustainability. New methods, created at the interface between photobiology and architecture, predicting the non-visual response to light are needed to support design decisions.

2013

Prospective evaluation of the Circadian Efficacy of (Day)Light in Rooms

Marilyne Andersen, Steven W. Lockley

Recent studies have attempted to link environmental cues, such as lighting, with human performance and health, and initial findings seem to indicate a positive correlation between the two. Light is the major environmental time cue that resets the human circadian pacemaker, an endogenous clock in the hypothalamus that controls the timing of many 24-hour rhythms in physiology and behavior. Insufficient or inappropriate light exposure can disrupt normal circadian rhythms which may result in adverse consequences for human performance, health and safety. This paper addresses the problem of prospective analysis of building architecture for circadian stimulus potential based on the state of the art in photobiology. Three variables were considered in this analysis: lighting intensity, timing, and spectrum. Intensity is a standard design tool frequently used in illuminating engineering. Timing and spectrum are not commonplace considerations, so the analysis that follows proposes tools to quantitatively address these additional requirements. Outcomes of photobiology research were used in this paper to define threshold values for illumination in terms of spectrum, intensity, and timing of light at the human eye, and were translated into goals for simulation – and ultimately for building design. In particular, the climate-based Daylight Autonomy (DA) metric was chosen to simulate the probabilistic and temporal potential of daylight for human health needs. The developed method was applied to study the impact of key architectural decisions on achieving prescribed stimulus of the circadian system in a hospital patient room design; studied variables included orientation, window size, and glazing material. A healthcare setting was specifically chosen with the intent of follow-on research to validate our findings with actual patient outcome data.

2008

Healthy lighting: integrating non-visual responses to light into building simulation

María Lovísa Ámundadóttir

2013