LOS ANGELES (CNS) — More Americans are getting fewer hours of sleep, according to findings released by researchers at USC and two other universities.

The study, published in the journal Sleep, found that more Americans reported inadequate sleep over time, with the sharpest increases among black and Hispanic adults.

The percentage of Americans reporting inadequate or “short” sleep — defined as six hours or less — increased from 29 percent to 33 percent of adults from 2013 to 2017, the study found.

“Poor sleep is a canary in the coal mine,” said Jennifer Ailshire, a USC assistant professor of gerontology and sociology and a lead author of the study. “We will see worse health outcomes as a result and we may be seeing that already.”

Researchers from USC, Arizona State University and the University of South Carolina examined data from the National Health Interview Survey for nearly 400,000 U.S. adults aged 18 to 84 from 2004 to 2017.

Respondents were asked how much they slept in a 24-hour period on average. Those responses were divided into three categories of short sleep (six hours or less), adequate sleep (seven to eight hours) and long sleep (nine or more hours).

The study’s authors found that the prevalence of short-sleep duration was relatively stable from 2004 to 2012, with an increasing trend toward short sleep beginning in 2013 that continued through last year.