How Circadian Lighting Uses LED to Improve Human Responses
Image Source: kanyanat wongsa/shutterstock.com
By Adam Kimmel for Mouser Electronics
Edited July 16, 2020 (Published April 21, 2020)
Introduction
The “Circadian” cycle, termed for the Latin circa dies (about day), references the human
body’s natural tendency toward alertness and drowsiness patterns that repeat about the same times each
day. The cycle is influenced by the regularity of sleep, consistency of activities/behaviors, and light and dark
conditions. Interruptions to the pattern of sleep disrupt the body’s circadian rhythm, leading to
increased weariness throughout the day. Although humans can not practically sync each day to their natural
rhythm, using circadian lighting helps reset a disrupted cycle to the natural frequency.
Circadian lighting dynamically adapts the intensity and color of light throughout the day to adapt to the human
circadian rhythm. This flexibility reduces the interference induced by other harmful factors. These benefits are
primary motivations behind the medical industry adopting circadian lighting to improve the patient experience
for newborns, long-term care patients, and patients with lengthy surgical recoveries.
Several mechanisms control the various settings of circadian lighting. LED component suppliers have leveraged
these levers and improved the control and reliability of lighting sources. Spectral engineers introduced
innovative features that create the phases of the circadian cycle within a single component. These phases
simulate the condition of the sun throughout the day and its brightness during periods of high and low
alertness. Here, we’ll review the control mechanisms used to produce circadian lighting, and a study of
how LED spectral engineering enables the entire cycle within an individual component.
Control Mechanisms
Color temperature, tuning, and rendering
The relationship between temperature and wavelength dictates the pattern of circadian light. Incandescent light
bulbs with a standard tungsten filament provide the basis for the light-color chart, which defines the range of
colors for circadian light.
Light color ranges from warm to cool, though “cool” colors (blues) correlate to higher temperatures,
6000K-7000K. Warm colors (oranges and yellows) correspond to lower temperature values, around 3000K-4000K. The
reason for this is the inverse relationship between the wavelength at which a black body emits the most light
and its temperature (Figure 1). The higher temperatures of the blue daylight colors carry the
shortest visible wavelengths; oranges and yellows have the longest ones.
Figure 1: Spectral intensity of Planck’s black-body radiation vs. wavelength
at various temperatures. (Source: Darth Kule)
With those relationships in mind, circadian light uses filament temperature to calibrate the color and intensity
to the desired time of day, depending on the geographic region. Generally, the color temperature increases
toward the middle of the daylight hours, peaks at noon, and decreases throughout the remainder of the day. Color
rendering, a process that tunes the test article to a black body source (under 5000K), checks that the filament
temperature matches the desired color of the corresponding time of day.
Contrast ratio and daylight tuning
A comparison of the highest color to the lowest for a given source, the contrast ratio defines the starting and
ending points of the circadian lighting scale. Lighting the light sequence to match the environmental conditions
and adapt for the time of year and daylight savings time while correlating with the typical human response
throughout the day. This process is the method for how circadian lighting syncs the person with a 24-hour cycle.
Intensity tuning, brightness/dimming, and stimulus tuning
Intensity tuning is a more straightforward approach to color or daylight tuning. Here, the color of the filament
remains constant, and the intensity of the light is increased and decreased through prescribed dimming steps.
Just as the temperature increases as the time moved closer to noon with color tuning, the dimming decreases
closer to midday, emitting a higher amount of light. Combining intensity and color tuning creates a custom
natural light pattern that engineers optimize for the circadian cycle.
LEDs and spectral power density (SPD)
LEDs are much easier to control than incandescent bulbs, and they enable spectral engineering to incorporate the
behavior of a circadian lighting system within a single component. Up to 80 percent more energy-efficient than
incandescent bulbs, the peak radiation temperature for a given color is slightly lower in LEDs because of a
higher percentage of input being converted to light. Figure 2 correlates the color temperature
of an LED to that of a traditional incandescent bulb:
Figure 2: The image shows the scale for LED and traditional bulb colors.
(Source: Milagli/Shutterstock.com)
Engineers use the Spectral Power Density (SPD) to control the intensity of light at given wavelengths along the
circadian cycle. Figure 3 below correlates the wavelength to various times of the day.
Figure 3: The image shows wavelength as a function of time of day. (Source:
Mouser Electronics)
From the wavelength image in Figure 3, the idea circadian lighting system would have the highest
intensities at the shortest wavelengths (450nm-475nm) around midday when the user needs to be alert. The system
would have the highest intensity at the longest wavelengths (600+nm) as the user sleeps.
Engineers can tune LEDs to a specific color temperature much more accurately than they can with traditional
lighting solutions. This feature unlocks the opportunity for a single smart device that lighting designers can
program to the desired times of the day. A sensor can capture the outdoor lighting state and transmit that
information to the controller, which then adjusts the LED to the given temperature. Various bulbs that operate
during the different parts of the day could be turned on or off after receiving the signal from the sensor. The
controllability of the LED ensures that it delivers the color temperature it was programmed to achieve
accurately.
Conclusion
Environmental conditions affect the human response. The body emits cortisol when awake and melatonin when asleep.
These chemicals are the body’s way of regulating how to spend its energy. People’s daily lives,
schedules, and social habits sometimes disrupt the body’s natural desire to follow the circadian cycle,
however.
Circadian lighting is gaining popularity in medical applications such as hospital in-patient, newborn, and
long-term care as a way to counteract that negative societal trend. Programming indoor lighting patterns
re-aligns the human cycle to the natural daily cycle set by the sun. This step allows the user to rebalance the
chemical release in the body and improves the enjoyment he feels at each phase by helping the body feel how its
best throughout the day.
Using LEDs and spectral engineering to tailor the circadian lighting cycle to the natural solar pattern
significantly improves the human response. With the addition of sensors, the device contains more of the
functionality of the system, reducing the number of components, size, and can be hidden better from the user.
Incorporating smart technology into the device would allow the circadian lighting solution to learn the
user’s pattern and individualize the experience even more with less manual operation. Ultimately,
the improved, improved control, smart technology, color options, and sensor inclusion will aid spectral
engineers in innovating a new platform of circadian lights.
Author Bio
Adam Kimmel has nearly 20 years as a
practicing engineer, R&D manager, and engineering content writer. He creates white papers, website copy,
case
studies, and blog posts in vertical markets including automotive, industrial/manufacturing, technology, and
electronics. Adam has degrees in Chemical and Mechanical Engineering and is the founder and Principal at ASK
Consulting Solutions, LLC, an engineering and technology content writing firm.
