When it comes to design, what is a healthy dose of daylight?

Circadian effectiveness w/ black line - Kyle Konis

Mapping circadian effectiveness through software: Red arrows indicate areas where daylight levels achieve a minimum threshold for 5 or more days a week all year, while black squares are never sufficient to support circadian rhythms.

Researchers are trying to improve the well-being of building occupants by measuring and maximizing a project’s circadian-effective area

Architects already know that access to daylight is important for the well-being of building occupants. Yet questions of “where,” “when,” and “how much” are becoming critical in the wake of emerging research showing the significant impact of light on the human circadian system.

The term “circadian” derives from Latin’s “circa dies,” or “about a day.” The circadian system is the body’s internal biological clock that regulates most 24-hour behavioral and physiological rhythms, such as sleep/wake, alertness level, hormone suppression/secretion, and core body temperature. Similar to the human ear, which supports both hearing and balance, the eye is responsible for vision and for keeping the circadian clock synchronized with the astronomical 24-hour day. Compared to the visual system, the sensitivity of the circadian system is shifted to the shorter wavelength “blueish” light, closely matching the light spectrum produced by a clear blue sky. Exposure of the eye to bright light in the morning (6am – 10am) is the most powerful timing cue used to entrain the circadian clock.

In modern buildings, light is often provided by electrical sources that may be adequate for the performance of visual tasks but lack the appropriate spectral composition and intensity required to stimulate the circadian system. Over time, lack of exposure to appropriate patterns of light/dark can disrupt the circadian system, leading to negative outcomes including poor sleep, reduced alertness, and increased risk for a range of health problems. Although lighting manufacturers have begun marketing “color-tuning” lighting products designed to mimic the daily changes in light spectrum produced by the natural environment, there is little evidence that these systems will ever serve as an effective and reliable substitute for daylight.

Mapping circadian-effective area and zones of biological darkness

Researchers at the University of Southern California (USC) are working on a metric and software workflow to help architects maximize the circadian-effective area of a project using daylight during design. Unlike the daylight metrics used in green building rating systems, which evaluate the amount of light delivered to a horizontal workplane over the year, a circadian daylight metric must assess light exposure at the eye and must also be sensitive to the timing, intensity, duration, wavelength, and past history of light exposures.

There is currently not consensus for what the minimum healthy “dose” of light should be, how long it should be present each day, or what units should be used to measure it.

The WELL Building Standard’s circadian lighting design precondition implements a minimum threshold of 250 equivalent melanopic lux, which must be available for at least four hours each day and can be provided at any point during the day. The term “melanopic” refers to a new photometric measure of light intensity weighed by the sensitivity of the melanopsin-containing light detectors in the human eye.

For comparison, researchers at the Lighting Research Center (LRC) recommend exposure to a circadian stimulus of 0.3 or greater at the eye for at least one hour in the early part of the day. These threshold criteria are concerned with the spectrum and intensity of a light source and can be met using light from any combination of daylight and electrical light sources.

Visualizations of annual, climate-based daylight information show the daily and monthly variations of circadian effectiveness in a model classroom.

Many designers will use daylight strategies to meet emerging threshold criteria, supplementing with electrical lighting in insufficiently-daylit zones. Because daylight is a constantly changing light source, there is a need to know “when” and “where” daylight is effective for circadian stimulus on a daily basis over the course of the year.

To address this need, a technique has been developed to analyze and map indoor environments to understand spatial and seasonal changes in circadian-effective daylight. Results can be used to identify “biologically dark” zones, where sustained occupancy over extended time periods (e.g. regular workday schedules) may present a risk for disruption of the circadian system. The technique can also be used to quantify the circadian-effective area (CEA) of a project, ranging from 0 to 100 percent, to compare the performance of various design alternatives.

Assisting architects and examining health outcomes

Supported with a 2016 AIA Upjohn Award, the USC researchers are now at work improving the accuracy and usability of both the circadian daylight metric and software workflow. The software workflow will allow users to map CEA using both WELL’s and LRC’s criteria, and to adjust thresholds such as light intensity and duration for specific projects and user-populations.

Once complete, the software workflow will be shared as a free tool, enabling designers seeking to use daylight as the primary source for circadian stimulus to understand how a given project’s local climate and design features impact human circadian stimulus and entrainment over daily and seasonal periods. Case studies will be performed to demonstrate how the metric can be applied to inform the design of common building types such as schools, commercial office buildings, and healthcare facilities.

As part of their ongoing research in memory care communities in southern California, USC researchers are also applying the metric to evaluate real daylit spaces to compare daylighting performance with health outcome data collected from residents.

Daylight as a design driver for improved health

The provision of a window in a room does not ensure that the room is sufficiently daylit to support healthy circadian entrainment. The area-based circadian daylight metric can be used to understand “when” and “where” interior daylight is effective for circadian stimulus. In the early stages of design, the goal is to maximize the area achieving the highest entrainment quality grades (e.g. A and B) by adjusting the building form, massing, aperture size, and ceiling height, while balancing additional concerns such as functional efficiency, program requirements, and cost. In later stages of design, the metric can be used to inform the selection of building components (e.g. glazing and facade systems) and controls for automated shading systems.

Appropriate electrical lighting systems can then be specified to supplement the light stimulus provided by daylight, with emphasis placed on zones found to have little or no circadian-effective daylight. Analysis can also be performed on an existing portfolio of buildings to identify and map circadian-effective and biologically dark areas to inform decision-making regarding their use or need for renovation.

As knowledge of the relationship between light and health increases, a better understanding of the circadian performance of the indoor environment can help to ensure that designers deliver projects that effectively support the biological lighting needs of building occupants.

Kyle Konis, Ph.D., AIA is an assistant professor of architecture at the University of Southern California, where he teaches in the Chase L. Leavitt Building Science Program.

Image credits

Circadian effectiveness w/ black line - Kyle Konis

Kyle Konis, AIA

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