CITIES WERE A MISTAKE:

It's still commonly assumed that poor sleep is a symptom rather than a cause of other medical or mental problems, according to Troxel. Yet today we know poor sleep can also cause illness. People with sleep apnea suffer more cardiovascular disease and stroke, as well as increased inflammation, which may contribute to illnesses including heart disease, cancer and arthritis. For teenagers, one study has shown, each hour of lost sleep comes with a 23 percent increase in the risk of tobacco, alcohol or marijuana use and a 58 percent increase in suicide attempts. Insufficient sleep may even make people more vulnerable to viruses and less likely to benefit from a vaccine.

Black Americans have markedly higher rates of sleep apnea, at least partly because they are more likely to be overweight or obese. Without special devices, like this CPAP machine, to help them breathe, sleep apnea sufferers can wake dozens of times per night, leading to daytime sleepiness and other health issues.

But here's where the great sleep divide comes in. Over the years, researchers repeatedly have found evidence that people in poverty get less sleep than those with more money. In 2013, for instance, a large CDC survey found that 35.2 percent of people earning below the poverty level reported sleeping less than six hours in an average 24-hour period, compared with 27.7 percent of those earning more than four times the poverty level.

The disparities are even sharper among racial groups. A rigorous 2015 study involving both lab tests and self-reports from more than 2,000 US participants found that, compared with whites matched for age and sex, Blacks were five times as likely to sleep for shorter periods. Hispanics and Chinese Americans were roughly two times as likely to get fewer hours of sleep than whites.

Several economic, social and physical factors contribute to these differences and their related harm to health, school performance and productivity.

Merely living in low-income neighborhoods is a risk factor for poor sleep, for a slew of reasons that include more light and noise pollution and less access to green spaces. "It's said that your zip code matters as much as your genetic code," says Troxel, who has gathered evidence demonstrating that where people live affects their health.

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