November 19, 2015


Contributed by Associate Professor Mehlika Inanici

We all have days that we experience sleepiness during daytime, and/or find it difficult to sleep at night. If we travel across multiple time zones, we experience jetlag. A growing body of research in the past 15 years has shown that human eye functions in a dual manner: it facilitates vision, and it resets the internal circadian body clock to synchronize it with the 24 hour daily cycle, or with the local time. Circadian means “about a day”. Sleep/wake phases, alertness, core body temperature fluctuations, and production of various hormones (such as melatonin) have daily cycles. Our internal body clocks are intrinsically close to 24 hour cycle, but a wide range of the population have a slightly longer cycle (>24 hr), and others have a shorter one (<24hr). Therefore, the internal body clock requires environmental cues (specifically, well timed light : dark cycles) to synchronize it with the local time.

After a long history of studies and design practices that employ strategies for enhancing the effects of lighting on experiential and perceptual qualities of space, supporting visual comfort and visual performance, and providing energy efficient environments, we are now tasked with another aspect of the luminous environment: We need to understand the role of lighting on health and well being. As we receive new information from the medical and neuroscience fields, a clear need emerges for architects and lighting designers to consider the impact of lighting on circadian rhythms. After all, nowadays human beings spend about 80-90% of their time in built environments. We would have been entrained to 24 hr cycles naturally, if we did not.

The timing, intensity, duration, and spectral content of light at typical interior levels affect whether we have a well-functioning timekeeping mechanism. The human circadian system is most sensitive to light at 460 nm (blue region of the visible spectrum). Daylight is a full spectrum light source, which is rich in blue spectra. Daylight exposure during early morning hours (before 10:00 am) can sync the internal body clock of the wide range of the population by advancing it to the local time. Daylight exposure during the daytime (before 6:00 pm) may be instrumental in sustaining alertness.  It is necessary to understand the changing nature of daylighting throughout the day and seasons, so that we can make informed decisions about the size and location of daylight apertures, glazing types, color of interior surfaces, and furniture layout. High level and blue-rich light at nighttime can be disruptive for sustaining healthy sleep cycles, so it should be controlled through design decisions.

Part of the task is to disseminate the knowledge, but if we are to engage in circadian friendly design practices, we need tools to quantify the impact of light stimuli on both visual systems and circadian rhythms. I partnered with Martin Brennan and Edward Clark (ZGF Architects, LLP) to develop a multi-spectral simulation method that can be employed to design and analyze circadian lighting in built environments. The tool and the methodology allows the users to consider local skies, exterior context, glazing optics, surface materials, interior design, and viewer location in determining the visual (photopic) and non-visual (circadian) effects of lighting.


Lark Spectral Lighting (, is developed in Grasshopper/Rhino environment, the simulation engine is Radiance; and it is provided as an open source and freely available tool. The objective is to make circadian lighting analysis more accessible to architects, lighting designers, researchers, and other interested parties.


Early bird gets the worm, as well as the blue light…