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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.


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

Human Health

Healthy people are free of disease and are otherwise well. Buildings with good indoor environmental quality (IEQ) support the health of occupants via high quality lighting, thermal conditions, air quality, acoustics, ergonomic and functional features.

Berkeley Center for the Built Environment | Occupant IEQ Survey

Whole Building Design Guide | Enhance IEQ

Usable Buildings.co.uk

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Did You Know?

People in the U.S. spend about 90% of their time indoors.

Source: Environmental Protection Agency (1987). The Total Exposure Assessment Methodology (TEAM) Study.


Case Study

Flexible Workplace Design

AWL Workstations

Today’s workplaces are often in flux. Organizations change direction or develop new services. People move to new spaces and take on new responsibilities. Teams form and re-form. The spaces themselves are transformed to meet these new needs. These changes are much easier to accommodate, when the workplace design supports flexibility.

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