Here’s one explanation for why a third of Americans don’t get enough sleep (at least seven hours a night): They’re exposed to too much light. A forthcoming study offers a comprehensive analysis of American sleep patterns, and posits that outdoor lighting–and street lights in particular–are seriously impinging our shut-eye time.

Researchers took an existing sample of 15,863 people from 15 states, who’d been asked about their sleep habits. Then they geolocated each interview against data from the Defense Meteorological Satellite Program, which shows how much light people are exposed to at night. The analysis shows that people living in areas of more than 500,000 people are exposed to six times more light than people living in small towns and rural places.

The people living in the more light-intense places were significantly more “dissatisfied” with their sleep than those in less light-filled areas (29% to 16%). They were 6% more likely to get less than six hours a night. They were more likely to be tired during the day (9% to 7%), and more likely to sleep less.

Lead author Maurice Ohayon, a professor at Stanford University, describes the statistical difference between the two groups as “very strong” and he notes the gap is likely not the result of other factors (for example, that some people watch TV in bed, harming the chances of a good night’s sleep). “We controlled for a lot of factors and that is giving more value to our study,” he says.

It is obvious that we don’t need so powerful lights in the residential areas.

Experts who’ve studied the relationship between light pollution and sleep say we should take unnecessary light more seriously than we do. Ohayon says we should reduce the amount of blue light in street lights (the part of the spectrum most like daylight) and take away lights in residential areas where traffic safety isn’t such a concern.

“We can play on the intensity of the lights and their color,” he says. “It is obvious that we don’t need so powerful lights in the residential areas than in the areas of high car traffic. If we take more care of our sleep environment, we will be healthier, more awake, more efficient and probably [have] a better mood.”