Circadian Light

Visual light vs. non-visual aspects of light

Light has three different aspects:

• It allows us to see the things around us. Typically, the human eye can see wavelengths of the visible spectrum from 390 to 700 nanometers (nm).
• Light also provides us with information about our surroundings. An example of this is found in traffic lights; a red light informs the driver to stop while a green light tells the driver he/she may go.
• Recent research has shown that there is also a non-visual aspect of light that helps to regulate biological processes, including people’s circadian rhythms. Studies have shown that light needed to stimulate a person's’ circadian system - enough to synchronize it with the 24-hour day, is at least 10,000 times greater than the amount needed to stimulate a person’s visual system. The circadian system is maximally sensitive to short-wavelength (blue) light while the visual system is most sensitive to longer wavelength light.

As a result, just because a person may have enough light to see the things around them does not mean that they are receiving light of the correct spectrum or intensity of light to beneficially impact their circadian rhythm.

What are circadian rhythms?

Circadian rhythms are biological processes that are generated and regulated by a biological clock located in the brain. These biological processes include body temperature, digestion, release of certain hormones, and a person’s wake/sleep cycle. In the absence of external cues, circadian rhythms in humans will run with a period close to, but not exactly 24 hours (in humans, circadian rhythms cycle every 24.2 hours without exposure to light). If a person does not receive enough light, their circadian rhythms become desynchronized with the local day-night cycle.

Impact of light on circadian rhythm

The circadian system is stimulated when light enters the retina and elicits a physiological response - the suppression of melatonin. Melatonin is a hormone produced by the body under conditions of darkness that cues the circadian sleep/wake cycle, as well as other biological processes. Regardless of the actual time of day, the brain perceives melatonin as an indicator of night and consequently, a time to go to bed. Morning light that enters the eye resets the body’s circadian system.

Lighting exposure

Important light characteristics affecting the human circadian system include the spectral power distribution of the light source (amount and spectrum), timing and duration of exposure, and an individual’s history to light exposure.

• Spectral power distribution
  • The circadian system responds to light differently than the visual system. The circadian system is maximally sensitive to short-wavelength (“blue”) light, with a peak spectral sensitivity at around 460 nanometers (nm), while the visual system, as measured in terms of visual performance or acuity, is most sensitive to 555 nm.
• Timing and duration of exposure
  • Light in the morning resets our circadian system and synchronizes our circadian rhythm to the local time of the day. However, light exposure at night can suppress our melatonin production and cause us to become more alert, thus delaying our ability to go to sleep. In addition, while the visual system responds to a light stimulus very quickly (less than one second), the duration of light exposure needed to affect the circadian system can take minutes
• Individual history to light exposure
  • The short-term history of light exposure affects the sensitivity of the circadian system to light. The higher the exposure to light during the day, the lower the sensitivity of the circadian system to light at night.

Circadian Entrainment

The daily pattern of light and dark sets the timing of our biological clock, which alerts our body it is time to sleep at night and stay awake during the day. Without light, the human circadian clock runs with a period slightly greater than 24 hours. As an example, in complete darkness and for many blind people, the sleep–wake pattern, if not reset, can be delayed by as much as 15 minutes. Without getting enough light, a person could wake up 15 minutes later each day. Sustained morning light is needed to reset our biological clock so it can synchronize with the local time. If a person receives a sufficient amount of light to be awake when it is light and asleep when it is dark, their circadian system is entrained or synchronized with the local 24-hour day.

Human are exposed to various amounts of light throughout the day, but for most, a large portion of their light exposure comes from electric sources. Because interior lighting fixtures are typically designed to meet only visual requirements, an individual’s circadian system often receives insufficient stimulus during the day, resulting in circadian disruption. Fortunately, the benefits of circadian-effective daylight can be supplemented through electric lighting under certain conditions.

Design Guideline for Promoting Circadian Entrainment with Light for Day-Active People (UL 24480)

The first edition of the Design Guideline creates an optional baseline performance goal for indoor lighting specifiers and building owners to consider when seeking to create work spaces with circadian-effective lighting in addition to quality illumination.

For over 100 years, designers have designed indoor lighting primarily to accommodate illumination for tasks, essentially for the horizontal work surface. This publication furthers the understanding of lighting’s impact on human health and well-being by adding design guidelines for indoor light that enters the human eye (vertical photopic illuminance). Alternate approaches using different assumptions about the spectral sensitivity of the circadian system are also discussed.

The purpose of UL Design Guideline 24480 is to help specifiers implement lighting schemes to promote circadian entrainment for day-active and night-inactive people occupying commercial, educational, and industrial spaces. The three goals of the document are to:

For more information on UL24480 and circadian-effective lighting in general, please watch an online seminar presentation on the implementation of UL Design Guideline 24480 and read through the Frequently Asked Questions.

Additionally, view a series of short instructional videos providing information on the background and process for implementation and verification of UL Design Guideline 24480:

• Introduction to UL 24480
• Background on Lighting and Circadian Rhythms
• Value of Circadian Effective Lighting (Energy and Economics)
• Overview of UL24480 Implementation Process
• Selection of Lighting Equipment for Implementation of UL 24480
• Using the Circadian Stimulus Calculator
• Designing for Circadian Stimulus
• Verification of Lighting Design Results

Related Topics

Circadian light

Circadian rhythms are biological processes that are generated and regulated by a biological clock located in the brain. These biological processes include body temperature, digestion, release of certain hormones, and a person’s wake/sleep cycle. In the absence of external cues, circadian rhythms in humans will run with a period close to, but not exactly 24 hours (in humans, circadian rhythms cycle every 24.2 hours without exposure to light). If a person does not receive enough light, their circadian rhythms become desynchronized with the local day-night cycle.

Learn more about Circadian Light.

Daylight Sufficiency Goal

The daylight sufficiency goal specifies the amount of daylight needed to provide adequate light to perform typical tasks appropriate to each space, without additional electric lighting. It is measured in lumens or foot-candles. 

Find more from GSA's Saving Energy through Lighting and Daylighting Strategies resource. 

Daylighting

Daylighting uses natural daylight as a substitute for electrical lighting. While it will likely be counterproductive to eliminate electrical lighting completely, the best proven strategy is to employ layers of light - using daylight for basic ambient light levels while providing occupants with additional lighting options to meet their needs. 

An effective daylighting strategy appropriately illuminates the building space without subjecting occupants to glare or major variations in light levels, which can impact comfort and productivity.

In order to provide equitable access to daylight ensure the space is optimized to disperse daylight well. Locate private offices toward the core of the space and specify low workstation panels. Use glass walls and light-colored surfaces on walls and desks to disperse daylight throughout the space. In all daylighting strategies, it is important to consider glare and to take steps to minimize it. Find more strategies below:

GSA | Saving Energy through Lighting and Daylighting Strategies

DOE LBL | Tips for Daylighting with Windows

Healthy Buildings

Health, as defined by World Health Organization in its 1948 constitution, is “a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity”. This definition of health has been expanded in recent years to include (1) resilience and the ability to cope with health problems and (2) the capacity to return to an equilibrium state after health challenges.

These three health domains - physical, psychological, and social - are not mutually exclusive but rather interact to create a sense of health that changes over time and place. The challenge for building design and operations is to identify cost-effective ways to eliminate health risks while also providing positive physical, psychological, and social supports as well as coping resources.

Learn more about Buildings and Health.