Outdoor lighting serves a purpose. We illuminate highways, public parks, runways and suburban cul-de-scas so people feel safe, stay alert and don't miss the sign for Cracker Barrel. But, lighting up the night sky comes with downsides, including (shocker!) sleeping issues. You can turn your bedroom into a resting oasis (with black-out shades, white noise, green-space views and no blue light). But, you can't necessarily control the world outside your 10-by-14 quarters. People who live in brightly lit pockets of America struggle with sleep more than those of us dozing by moonlight, according to a new study from Stanford University and NASA. The findings appear this month in the journal SLEEP.
For the study, researchers looked at the relationship between Americans' sleep habits and their exposure to outdoor nighttime light. To do this, they borrowed data from national, multi-year phone surveys of nearly 16,000 adults living in 15 states. Participants answered questions about their sleep complaints, patterns and conditions (i.e., when, how well, how long, how sleepy, where, with whom, in what lighting), as well as their demographic stats, living conditions, lifestyle habits and jobs.
With a little help from Google Earth, researchers then assessed levels of light exposure based on geographic coordinates. The Defense Meteorological Satellite Program (DMSP), which "monitors meteorological, oceanographic and solar-terrestrial physics for the US Department of Defense," has measured nighttime light levels since the early '70s. Nighttime light levels in heavily lit areas, e.g., dense cities, highway-adjacent developments and strip-mall parking lots, are exponentially higher than those in areas running on moonlight. On full-moon nights, light levels max out around .3 lux. By contrast, a standard-issue street light registers around five lux and a parking-lot light gives off 20. Of course, that's only one light and parking lots have rows of 'em.
Across the board, people exposed to high levels of outdoor nighttime light reported symptoms consistent with circadian rhythm sleep disorder, including delayed bedtimes and waketimes, and short, unsatisfying rest.
The results underscore the importance of light exposure for sleep health, but they shouldn't be surprising. We know that gazing into artificial light (regardless of the source) suppresses the release of melatonin, the drowsy-making hormone that influences sleep-and-wake schedules via screwing with circadian rhythms. Lab studies, researchers wrote, have shown that exposure to fairly low-intensity lights can delay circadian timing and minimize sleepiness at bedtime. So, living in the constant presence of bright light could reasonably take a toll. With that said, most of the US (currently) lives in urban areas, where outdoor light shines all the live-long day.
But, researchers caution against making too much of the findings. "It has to be kept in mind that this study is purely observational: We found several significant associations with outdoor nighttime lights and sleep behaviors but because of the nature of this study, actual level of lights could not be assessed," they wrote. Adding: "We did not ask for the presence of curtains in the bedroom windows and the opacity of the curtains nor for the use of a sleeping mask."