Lighting Better Lives
By: Craig DiLouie

The biggest impact light has on human health is the circadian system, which regulates body functions based on circadian rhythms that are in turn based on the 24-hour day/night cycle. As a key example, exposure to light affects the timing of hormones like melatonin, which regulates sleep. The light enters the eye through special photoreceptors tied to the brain’s master clock. In the modern age, electric lighting plays an even more important role in circadian rhythms than daylight. The uncertainty of light exposure can disrupt these rhythms, which can lead to poor sleep and negative health effects. As a result, the lighting industry is beginning to show increasing interest in lighting design that satisfies visual needs and promotes circadian health.

When it comes to light and health, what do we know, what don’t we know, and what is actionable?

Marianna Figueiro, professor and light and health program director at the Lighting Research Center at Rensselaer Polytechnic Institute, said lighting that is well designed to support circadian health can promote not only alertness and reduction in feelings of sleepiness during the day, but also improved sleep for people with Alzheimer’s disease. She added that there are four main characteristics that affect light’s impact on circadian health: intensity, spectrum, timing, and duration.

  • Intensity refers to the amount of light falling on the eye’s photoreceptors each day. That means vertical, not horizontal, light levels. She explained that a light level of about 30 to 40 vertical footcandles  (about 80 to 120 horizontal footcandles) is desirable in the morning. In the evening, starting about two hours before bedtime, one to two footcandles is preferable.

Providing 80 to 120 horizontal footcandles is at odds with current practice and energy codes. Providing 30 to 40 vertical footcandles entails lighting vertical surfaces such as cubicle walls (daylight is ideal). Task lighting can help, as can lighting controls.

  • Spectrum refers to the wavelength of the light. Short-wavelength light (specifically 460nm, associated with the color perception of blue) produces the best circadian response. According to Figueiro, spectrum may increase circadian response by a factor of two, but intensity is more important; spectrum alone may not be enough. Lighting controls that allow color adjustment can be helpful. 
  • Timing refers to when light is received by the eye. Morning light encourages an earlier bedtime. Evening light may delay sleep. People working different shifts in a 24-hour facility will have different requirements. 
  • Duration refers to the amount of time of exposure. While intensity is important, what is most important is the cumulative amount of light falling on the eye during the day.

These mechanisms are becoming clear and allow lighting practitioners to design lighting systems more conducive to circadian health. However, other factors come into play, such as nighttime light exposure and lifestyle. As a result, lighting should be considered as playing a supporting role in supporting circadian health. Ideal applications in which to start include facilities occupied 24/7 on a predictable schedule, such as assisted-living facilities and potentially schools. 

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