What is Circadian Lighting?
The experts at the Lighting Research Center say: “When specifying lighting for the circadian system, more specifically, for circadian entrainment, it is important to consider light level, spectrum (color), timing and duration of exposure, and photic history (i.e., previous light exposures).”
Color is one aspect of lighting for circadian entrainment, but it hardly tells the whole story. The goal of this article is to clear up misconceptions about circadian lighting by explaining the importance of light intensity, location, duration, and previous light exposures as they relate to architectural lighting’s potential impact on our health and well-being.
The intensity of light (light level) that enters your eyes is the primary variable that impacts your circadian rhythm. It does not matter if the light looks warm or cool, bluish or yellowish, 2,700 Kelvin or 6,500 Kelvin, if you have enough intense light entering your eye it will suppress your melatonin. Take a look at the graph below developed by the Lighting Research Center.
From a high level, it tells us about how different intensities of light impact our melatonin suppression. For example, outside during the day we can expect the maximum melatonin suppression of 70%. That makes sense because outside during the day time is when the most
intense light will enter our eye. Indoors at night, there are low light levels, and a correspondingly low suppression of melatonin. Notice as well, there is nothing about light color temperature…
But what about the range of 450-490 nanometer wavelength that is proven to be particularly efficient at suppressing melatonin production? It is true, the light sensing cells (Intrinsically Photosensitive Retinal Ganglion Cells) inside our eyes are particularly sensitive to that specific range of short wavelength bluish looking light. However, intensity still takes precedent. If you have light tuned to that optimal wavelength range, but it is at only 10 lux (low intensity per area), it is not going to suppress your melatonin as well as light at 300 lux (high intensity per area).Take a look at the chart below from the LRC. We are only slightly more sensitive to cool light.
As you can see from the LRC’s research above, we have to consider the amount of light entering our eye more than the color of the light.
The angle at which light enters your eye can cause significant variations in how affective light will be at suppressing your melatonin. For example, if you are looking towards the sun, you are receiving far more intense light into your eye than if you were to look away from the sun. The same principle applies indoors. If your lighting is coming from the ceiling above, far less light will enter your eye than if the same light was delivered from the wall in front of you.
We have evolved over millions of years with the sun, which arrives every morning on the horizon to the East, spends a short time overhead at midday, then leaves at the other horizon to our West. Because of this, our IPRGCs are more sensitive to light that hits the bottom of the back of our eyes.
In our built environment, most of our lighting is directly overhead for the entire day, and is not optimized to impact our circadian rhythm, even at high intensities. By delivering light from the vertical plane (as shown with the clerestory lighting application below), it becomes easier to provide a circadian stimulus without needing 100% intensity and the coolest color temperature. Of course, it is also important to account for light reflecting off of other surfaces and entering our eye, but we will not go further into that topic here.
For those of you who are striving for WELL Building Standard certification, it will be very challenging to achieve the lighting concept credits if you are lighting only from the ceiling. Many lighting designers have shared their frustrations with needing to have the lighting programmed to 6,500 Kelvin and 100% intensity for hours every day to achieve the required amount of Equivalent Melanopic Lux . At the same time, it is necessary to control energy consumption, visual comfort, and glare. By delivering light from the wall, you are creating an unobstructed
path for light to enter the eye. This means that you can achieve the required Equivalent Melanopic Lux with lower overall intensity.
“Lower light levels, such as those that might be specified for horizontal illuminance in a given workspace, will achieve relatively lower CS values unless compensated for by an SPD with more power at shorter wavelengths.”
If you are in an intense, perfectly angled lighting environment for 30 seconds, the light will not have time to significantly impact your melatonin. The LRC circadian stimulus measurements suggest that it is possible to achieve saturation, or 70% melatonin suppression in one hour. Also, experiencing intense light earlier in the day is associated with sleep and mood benefits. Perhaps someone will figure out how an instant flash of light can create lasting effects on our circadian rhythm, but that technology does not yet exist as far as I know. One takeaway from this should be that it may not make sense to invest in circadian lighting in spaces that people will not spend an hour or more at a time.
“Our research shows that exposure to a CS of 0.3 or greater at the eye, for at least 1 hour in the early part of the day, is associated with better sleep and improved mood. ”
Photic History & Challenges
I hope this explanation of the latest research on light’s impact on our circadian system helps you understand the key variables of specifying lighting for circadian entrainment. Intensity, Duration and Location, and light color all have an impact on our circadian rhythm. However, if we are truly going to make a positive impact on people’s lives, a significant obstacle still remains unaddressed. We are all unique, and the way we spend our time changes from day to day. If I go on a run at 6AM in the summer when it is light out, I have different lighting needs for the rest of the day than if I were to wake up and check my email and go right to the office. This can change day to day for me, and is a particularly challenging issue for shift workers who may work three night shifts per week. A lighting environment optimized for a child requires different lighting conditions than an environment for an elderly person. The elderly tend to need more light for a variety of reasons. So you can begin to see how formidable this challenge really is. At an elementary school, you have teachers of a variety of ages in a room with a bunch of seven year olds. In a hospital, you have patients and staff of all ages, ethnicities, genders, and lifestyles. Some of them are working twelve hour night shifts, and others are sleeping regular night hours. What do we do as architectural designers and product designers?
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