Can sleep explain why black Americans die younger than white Americans?

The racial inequalities in the US are stark, but none are more damaging than the health gap between blacks and whites. On average, blacks die at a significantly younger age than whites.

The trouble is that, while we’ve known this for more than 100 years, we still don’t really understand why. James Sherman, an expert on this health gap, says, “Despite efforts to uncover and elucidate the reasons for this persistent gap, we have achieved only limited insights.”

Now a growing body of evidence is making researchers wonder whether variations in sleep patterns may go some way to explain the health gap. Sleeping too little or too much increases the risk of cardiovascular disease. People who were sleep-deprived for one to three weeks performed much more poorly on cognitive tasks than those who slept well. In 2014, the US Centers for Disease Control (CDC) declared that insufficient sleep has become a “public-health epidemic.”

In its report, CDC highlighted that as a proportion of the population, blacks slept much less than all other races.

Other factors such as income inequality, discrimination, and racism certainly contribute to the health gap, but sleep could help explain how those problems translate to worse health outcomes. And if the explanation is right, it can have huge implications in how to overcome this disparity.

Better late than never

If sleep is indeed the missing link that can explain the racial health gap, it took researchers a long time to find it because only in recent decades has sleep been considered a crucial part of human health.

And even after people realized sleep’s importance, most studies have been based on self-reported surveys, which can be a flawed way of accurately measuring real trends. Increasingly, researchers have realized that poor data may be causing them to overlook an important aspect of human health.

There are two ways to overcome the poor data on sleep: Either measure sleep more objectively by asking people to wear medical devices that record their sleep; or improve data collection by asking a much larger number of people to answer the survey, and do it in a way that reduces inaccuracies in sleep-reporting data. A recent episode of the Freakonomics podcast highlighted two sets of studies that tried to overcome those limitations, and their implications are startling.

In one study, led by Diane Lauderdale at the University of Chicago, 669 participants—half white and half black—wore wrist actigraphs that measured their activity through the night and how long each participant actually slept. The upshot: People are not very good at judging how much actual sleep they get. The actigraph-measured sleep was, on average, only six hours, compared to the seven hours that the participants reported in a survey.

Women slept more than men and, intriguingly, white people seemed to sleep significantly more than black people. White women slept nearly an hour more than black women (6.7 hours vs 5.9 hours), and white men slept slept an hour more than black men (6.1 hours vs 5.1 hours). White women slept nearly 90 minutes more each night than black men.

These findings are worrying because sleeping less than six hours can have dire health consequences. (There is only a tiny, genetically-gifted proportion of the population that can get away with such habits without adverse effects.)

The other study approached the question differently. It made use of a huge survey—called the American Time Use Survey (ATUS)—in which 150,000 people were asked questions about how they used their 24 hours. When people describe their time use throughout a full day, they are less likely to misreport their sleep hours, according to Dan Hamermesh at the Royal Holloway University of London, who analyzed the survey.

Hamermesh’s initial analysis went against what Lauderdale had found. He found that, after adjusting for education, age, marital status, presence of kids, and immigrant status, US blacks sleep more than whites on average, whereas Lauderdale had found the opposite.

But when he re-ran his analysis looking for extreme sleepers—those who slept less than six hours or more than 11 hours—he found results consistent with Lauderdale’s findings: Blacks were much more likely to be extreme sleepers than whites. Thus, extreme hours were skewing the average and making it look as if blacks were sleeping more than most were.

The missing link?

So does sleep really provide the missing link to why black Americans die younger than white Americans? The racial health-gap expert Sherman thinks it’s plausible.

He pointed out that while sleeping our blood pressure drops and it provides the cardiovascular system an opportunity to recover. “If one’s blood pressure is higher, even while you are sleeping, then that’s going to add to your long-term risk for heart disease,” he says.

But any claim that sleep is the cause of the racial health gap must come with significant caveats. These are correlational studies showing an implication that poor sleep can cause poor health. However, the opposite can also be true: Poor health causes poor sleep, and the causes of poor health could be something else.

Lauren Hale, a sleep researcher at the State University of New York at Stony Brook, argues that a growing body of evidence proves that sleep deprivation in itself affects health, and that there is reason to believe that sleeping too much can also be detrimental to health.

Hale’s own studies give her confidence that variations in sleep can explain some of the racial health gap. She is involved in a project called the Fragile Families and Child Wellbeing Study, which has 5,000 registered children born between 1998 and 2000 to mostly unwed parents. Hale’s initial findings show that black children are sleeping less than white children, and they have fewer bedtime routines.

Crucially, she says, “We found that cognitive outcomes were higher among children who had a regular, language-based bedtime routine, reading a story, or singing a song.”

The reason blacks are sleeping less may come down to to several factors. Blacks are, on average, poorer than whites. This means some cannot afford the things required for sleeping well: a good bed in a quiet house.

Also, recent research shows that those who face discrimination on daily basis produce greater level of stress hormone cortisol. Among blacks, the level of cortisol remains much higher in the evenings than among whites, and cortisol’s interference at these hours leads to poorer sleep.

The science of sleep still has some ways to go, and more research will be needed to confirm the connection. But, if true, it might provide another way to overcome a shameful health gap in the United States.

Read next: Why black Americans can’t sleep at night: racism

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