The number of Americans sleeping six hours or fewer each night is on the rise. According to a new survey of nearly 400,000 individuals’ sleep patterns, 32.9 percent of respondents self-reported a poverty of zzz’s in 2017, up from 28.6 percent of respondents in 2004.

The findings, published in the journal Sleep, reflect a worrying trend of national sleep deprivation, specifically among African-American and Hispanic respondents. As Rodrigo Pérez Ortega writes for Science News, both groups report higher rates of inadequate sleep than their white counterparts.

Of those polled in 2017, 40.9 percent of African-Americans, 32.9 percent of Hispanics and 30.9 percent of Caucasians said they typically slept fewer than six hours per night. The rapid increase evident in these figures—up 6.5 percentage points amongst African-Americans and 7 percent amongst Hispanics from 2004—means these groups are not only suffering from a more severe sleep surfeit, “but at a faster rate over time,” says study co-author and University of Southern California gerontologist Jennifer Ailshire. From 2004 to 2017, the comparable percentage of Caucasians reporting so-called “short sleep”—six hours or fewer a night—increased by about 2 percent.

“Poor sleep is a canary in the coal mine,” Ailshire tells USC News’ Jenesse Miller. “We will see worse health outcomes as a result.”

Per the National Sleep Foundation, adults aged 18 to 64 should sleep between seven to nine hours per night. Adults over 65 can skim off an hour, enjoying a recommended seven to eight hours of sleep. But many Americans regularly fail to meet this quota, leaving them vulnerable to short-term consequences such as an inability to concentrate, mood swings and issues with memory. On a day-to-day basis, a lack of sleep can also raise the likelihood of getting into an accident or engaging in conflicts at work and at home. Over time, the built-up effects of sleep deprivation may lead to weight gain, a weakened immune system that increases the risk of developing diabetes and heart conditions, and even dementia.

Lead study author Connor Sheehan, a demographer at Arizona State University, tells ASU Now that theories regarding the rise in short sleep may stem from increased stress levels and an uptick in smartphone usage before bedtime. While Sheehan says that the study controlled for income and education variables, he speculates “it’s possible that an increase in race-related discrimination, police violence and focus on deportation from 2013 to 2017 had an effect” on African-American and Hispanic participants.

Sheehan outlines several good bedtime habits in his Q&A with ASU Now: Refrain from using your phone or watching TV directly before bed, don’t over-eat prior to bedtime, and keep your room at a comfortable temperature. Most importantly, he says, make your bed a space dedicated to sleep rather than a spot your brain associates with daytime activities.