A study of more than 11,000 Americans found that sleeping too little or too much slightly raises the chance of death by creating imbalances in the immune system, by way of changes in white blood cell counts.
In a paper published this month in American Journal of Human Biology, researchers at the University of Texas at San Antonio were the first to find evidence of a throughline connecting sleep, white blood cell counts and death.
Scientists have known for some time that regularly sleeping for more or less than roughly seven to nine hours per night is linked to higher mortality and a long list of health conditions, and that abnormal sleep alters numerous components of the immune system. But the specific pathways and mechanisms through which sleep habits interact with the body and cause mortality risks aren’t fully known.
“If your immune system is being affected and outside of its optimal window, that can lead to increased mortality risk down the road — we know that from previous literature,” said Eric Shattuck, a biological anthropologist and the study’s lead author. “We've just put all these pieces together to show that pathway from sleep to immune function to mortality does show up.”
He and demography professor Corey Sparks took subjects’ data from a large annual health survey taken between 2005 and 2011 and compared their average sleep per night on weekdays, their counts of white blood cells and whether they died between the time of the survey and 2014.
Compared with the surviving participants, the 2% of people who died had slept longer and had higher counts of white blood cells, and part of sleep’s effect on the risk of death was mediated by its changing of cell counts. They also had lower counts of monocytes — a kind of white blood cell — although the relationship disappeared after adjusting for other factors.
The effects found in the study were statistically significant but small and uncertain, with abnormal sleep making dying at most 0.51% more likely, and its changes to white blood cell counts altering the chances by about 0.01%.
Shattuck acknowledged that the study’s limits left many sources of uncertainty, such as imprecise sleep data and an inability to fully capture the immune system’s complex interactions in the body. Despite the uncertainty and small effect sizes, the causal link between sleep, the immune system and mortality is a new and important finding, he said.
“We took great pains when writing this paper to make sure that we're not claiming that, like, ‘Hey, if you get 10 hours of sleep, your immune system is going to go crazy and you're gonna die,’” Shattuck said.
The study's limitations may have also contributed to a contradictory result the analysis produced. After accounting for variables such as body mass index, age and other demographic data, Shattuck and Sparks found that above-average sleep lowered mortality risk, a conclusion that goes against their own analysis of a related dataset and substantial sleep literature.
The data was taken from the National Health and Nutrition Examination Survey, or NHANES, which is intended to be representative of the U.S. population. The yearly survey from the Centers for Disease Control and Prevention allowed the researchers to track a large number of people over several years and study their death rates.
“A lot of the existing research that's done on sleep and immune function tends to be in smaller samples,” Shattuck said. “You're getting, let's say, 20, 30 people as the higher end, asking them to forgo sleep or to record their sleep.”
The NHANES data had its own limits, as sleep and mortality data were both available during only the six years analyzed in the paper. Participants self-reported their sleeping hours, a measurement less precise than laboratory monitoring.
The researchers noted that they do not know whether the mortality effects hold in societies with different sleep patterns. Both the Malagasy in Madagascar and the Hadza in north Tanzania appear to have segmented sleeping patterns, sleeping six-and-a-half hours or less per night and napping most days. Because many in these societies practice “traditional” hunting and gathering and do not use electricity, they could provide insight into the sleep habits of early humans.
Better understanding sleep biology in these indigenous societies could shed light on “population variation in, and the evolutionary history of, human sleep patterns,” the researchers wrote. Shattuck said he and sleep expert David Samson at the University of Toronto have plans to work alongside the Wixárika people in Mexico to study their sleep and health.
The article, “Sleep duration is related to increased mortality risk through white blood cell counts in a large national sample,” was published Feb. 1 in American Journal of Human Biology. The authors of the study were Eric Shattuck and Corey Sparks, University of Texas at San Antonio. The lead author was Eric Shattuck.