Just because you're tired, doesn't mean you didn't get a full night's sleep.
We’ve been told that the modern, connected life is taking a toll on our sleep. Compared to previous generations, studies report, we’ve been sleeping less and less every year. And that is making us “more likely to suffer from chronic diseases such as hypertension, diabetes, depression, and obesity, as well as from cancer, increased mortality, and reduced quality of life and productivity.”
It sounds terrifying, but it’s probably not true. For a long time doctors and scientists had ignored sleep’s importance to health. We’ve only begun to see how much it matters in the last few decades. And thus, we have never systematically gathered data on how much people really sleep.
Now, researchers have started to put together what scant data we have to look at the bigger picture. And what they have found is that we aren’t sleeping any less today than before. Knowing precisely how much we sleep matters, because sleep plays a pivotal role in many aspects of our health—from staying mentally fit to fending infections.
In the 1980s, researchers began to probe how sleep affects health. A 1989 study set off alarms when researchers showed that rats deprived of sleep start dying in as little as two or three weeks.
By looking at the effects of sleep deprivation on humans, it’s been determined that the average adult needs seven to nine hours of sleep. When we sleep less than seven hours we have difficulty with memory and simple cognitive functions. (Though a tiny fraction of people can get away with much less.)
According to a survey conducted by the US Centers for Disease Control (CDC) between 2005 and 2007, more than 30% of adults slept less than six hours a night. The National Sleep Foundation’s own surveys reveal something similar: more than 20% of people in 2009 were sleeping less than six hours compared to only 12% in 1998. The CDC declared that insufficient sleep was becoming a public health epidemic.
The CDC announcement came at a time when doctors across the US were increasingly prescribing sleeping aids and sleeping pills. The number of adults on sleeping pills has tripled in the last decade alone.
However, a 2010 analysis, published in the journal Sleep, which used data from a different set of surveys conducted between 1975 and 2006, found very different results. It showed that the proportion of short-sleepers (those sleeping less than six hours) hadn’t changed much in the last 30 years. And, more surprising still, that proportion was only 9.3% in 2006.
Why such a large difference when compared to the CDC data? “Probably because those studies asked a different question,” Kristen Knutson, a sleep researcher at the University of Chicago who conducted the 2010 analysis, told Quartz.
For instance, in the case of the CDC survey the participants were asked, “On average, how many hours of sleep do you get in a day?”
Knutson believes that answers to such a question are likely to suffer from both conscious and unconscious biases, which may make people give a different answer than reality. Sleeping less, for instance, is associated with being more productive and some may consider it fashionable to say they sleep less. Some studies have also shown that people underestimate how much they actually sleep—especially those who suffer from insomnia.
A more effective approach is to ask how people spend their average day then tease out data about total time spent sleeping, which is what Knutson’s study does. It uses data from the American Time Use Survey (ATUS), which is conducted with input from more than 150,000 people.
Both the CDC and ATUS data only involve people in the US. If there were a cultural shift in sleep patterns, it likely would’ve occurred across the Western world and would be reflected in other countries too.
A 2012 systematic review of 12 studies from 15 countries, published inSleep Medicine Reviews, showed that, between 1960 and 2000, total sleep times across these countries hasn’t changed much at all. They increased by less than an hour per night in seven countries (Bulgaria, Poland, Canada, France, Britain, Korea and the Netherlands), decreased by less than 30 minutes per night in (Japan, Russia, Finland, Germany, Belgium and Austria), and showed no change in Sweden and the US.
When Shawn Youngstedt, a sleep researcher at Arizona State University, examined these studies, he realized that there might be a way to resolve these conflicting results. Instead of using self-reported data, he wanted objective data—that which is recorded using sleep-monitoring instruments or by observers as participants slept in a lab.
Youngstedt’s and his colleagues’ systematic review, published in Sleep Medicine Reviews, took into consideration 168 studies with objective data conducted between 1960 and 2013—involving more than 6,000 participants (understandably a much smaller set than the self-reported surveys) across 15 countries. It too reveals that the total sleep time hasn’t changed much in that period. Most of us sleep between seven and nine hours, and the proportion of those sleeping less than six hours hasn’t increased in the last 50 years.
Despite this, the CDC tells Quartz that poor sleep remains a public health epidemic. The differences in the studies, it says, “may have arisen from the different surveys used, different definitions of short sleep, as well as different statistical analyses conducted.”
“To call something an epidemic, you need an extraordinary amount of data supporting the claim,” Youngstedt told Quartz. “But the data just doesn’t seem to show that.”
So why then are there widespread worries of a poor sleep “epidemic”? It’s probably a combination of social trends that fuel the myth.
Sleeping is commonly considered a leisure activity, and the modern fast-paced life creates an illusion that we have less free time for rest. Cases of famous people succeeding on little sleep—from Margaret Thatcher to Marissa Mayer—make matters worse.
Some also believe that we must be sleeping less than our ancestors who never had access to electricity. The invention of the light-bulb did change our sleep habits, but not the total amount of time we slept. Before the 18th century, a segmented sleep pattern was common. People slept for four hours, then woke up for a little bit of time and slept for four more hours later.
Sleep wasn’t considered a crucial part of human health for a long time, and as a result not many well-designed studies have been conducted in the past. “So we may never definitively know how much people really slept then,” Youngstedt told Quartz.
His results are unlikely to represent all classes of sleepers. For instance, studies have shown that Black Americans probably sleep much worse than White Americans. It has also been proposed as a possible explanation for the wide health gap between the races. Also, his study only looked at healthy sleepers, and cannot say whether poor sleep increases risk of diseases such as diabetes and obesity or whether those at risk of disease suffer from poor sleep.
Though we seemingly sleep enough, scientists are finally starting to understand just how crucial how sleep is for good health. With the field of sleep research gathering pace and more of us strapping on wearable devices that can monitor our sleep, we are bound to learn more. While that happens, there is little reason to lose sleep worrying about not sleeping enough.