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Street Lights and Circadian Sleep Cycles

An Overview of Street Lights and Circadian Sleep Cycles

by Dr. Steven Lockley, Harvard Medical School, Division of Sleep and Circadian Disorders, and Sam Lipson, Director of Environmental Health, Cambridge Public Health Department, contributing

Our "circadian clock" is sensitive to all visible light, and any exposure to light after dusk moves us away from a natural cycle. The circadian system is more sensitive to blue light, but the intensity of light is at least as important when considering the impact of light.  Just because a light source appears “bluish” does not mean it is necessarily disruptive and just because it appears “warm” does not mean that it is not disruptive.  Light wavelength and intensity interact to determine the effect of light on circadian rhythms and sleep-wake cycles.

  • When considering the effects of light at night, indoor lighting is typically of more concern given the intensity and proximity of the light sources to the eyes, which detect the light. The quantity of light emitted by streetlights, both High-Pressure Sodium (HPS) and LED, is many times lower than that emitted by typical interior lights, TVs, tablets, or PC screens.
  • Studies on the effect of blue light and recent articles that warn about health issues associated with light exposure at night refer primarily to device screens and interior light sources as having possible negative impacts.  These risks are primarily based on lighting associated with night-shift work.
  • Research suggests that the light levels from streetlights (both HPS and LED) are simply too low to cause significant negative circadian or sleep health problems, especially if the light reaching the eye is further reduced by curtains, eye masks or closing the eyes to sleep. There is no evidence that typical exposure to street lights of any color is disruptive to the human circadian system.
  • These new LEDs are slightly “bluer” than the old HPS streetlights and are estimated to have about 20% more of the stimulatory blue wavelengths before the dimming program was implemented.
  • After dimming (at 8 p.m. or 10 p.m. in most areas), the stimulatory effects of the lights on non-visual responses (associated with alerting the brain or circadian disruption) are estimated to fall below that of the old HPS light by about 40%, reducing the risk of light disruption of sleep and circadian rhythms.
  • It is important to emphasize when interpreting these percentages that the absolute amount of light generated by the new LED street lights (or the old HPS street lights for that matter), that reaches the inside of residences, is very low and would not be expected to have a meaningful impact on alertness or sleep.
  • For those wishing to further limit effects of light exposure after dusk, we advise individuals to:
    • Avoid bright light in the evening and gradually dim or reduce light exposure through the evening as you approach bedtime
    • Limit use of electronic devices in the evening, and certainly within 30-60 minutes of bedtime (read a real book, not an electronic one before bed!)
    • Remove the TV and electronic devices from the bedroom
    • Try and use lower CCT sources in the evening (4000K or less)
    • Install dark, thick curtains or black-out curtains, particularly in the bedrooms
    • Consider using an eye mask during sleep
    • It is estimated that simply closing your eyes during sleep stops about 95% of all light sources from getting through the eyelids
    • Try and keep as regular a schedule as possible
    • Make sure you get as much daylight exposure as possible during the day

Again, these measures will help reduce the impact of light sources commonly used in the evening at home, which is of much greater concern than the much lower levels of light emitted by street lights. The health concerns of indoor light sources will easily override any effects of street lighting.

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