Smartphone Study Uncovers Why So Much of the World is Short on Sleep

Age, gender and nationality impact how much we sleep, and social pressures rob many of needed rest



How sleepy you are may depend on a number of factors, including age and where in the world you live.

Daniel Forger and Olivia Walch of the University of Michigan designed a free phone app, called ENTRAIN, that helps travelers overcome jet lag by creating optimized personal lighting schedules. The app is driven by a mathematical model that works effectively only when users accurately input such information as their location, sleep hours and daily exposure to light. The scientists, seeing potential in such data, asked users to anonymously volunteer the information collected by the app. Some 10,000 people from 100 countries did just that.

“It's pretty amazing that for almost no cost we ended up with, I think, one of the richest and most interesting datasets on human sleep ever collected,” Forger says. “The unsung heroes in this are all the people who agreed to send us their data.”

What they shared revealed some notable patterns, Forger and Walch, along with UM colleague Amy Cochran, report today in Science Advances. Some nations, for example, are home to night owls while others have citizens who enjoy more beauty sleep. Residents of Singapore and Japan clocked in at the low end, averaging only 7 hours 24 minutes of sleep per night. The Netherlands, in contrast, topped out at 8 hours and 12 minutes of sleep on average each night.

Women most everywhere seem to schedule about half an hour more sleep per night than men. “That's huge,” Forger says. “Half an hour actually makes a huge difference in terms of your overall performance.” Middle-aged men get the least sleep, on average, and often sleep less than the recommended 7 to 8 hours.

As people get older, though, their sleeping schedules look much more alike. “When we looked at the sleep habits of different age groups in our population, we noticed that the distributions of bedtime and wake time were getting narrower as age increased,” Walch notes. This may be real-world support for the results of past studies, she adds, that found that older people had narrower windows of time in which to fall asleep and stay asleep.

Paul Kelley, who researches sleep and circadian neuroscience at the University of Oxford, said it was encouraging to see technology and mathematical models applied to sleep science. “Inventive new methods and new outcomes may offer additional ways to understand our biological timing systems,” he notes, while cautioning that such research remains a work in progress.

Many people don't get enough shuteye. A recent CDC study found that one in three U.S. adults doesn't get the recommended seven minimum hours on a regular basis. And people in other nations around the world are similarly exhausted. That creates problems far more serious than grumpy breakfast conversations and coffee cravings. Sleep deprivation can boost one's chances of obesity, diabetes, high blood pressure, heart disease, stroke, stress and other ailments. And fatigue makes people perform all kinds of mental and physical tasks poorly, which is why sleep scientists keep suggesting that school days should start later.

A primary cause of all this missed sleep is the daily tug-of-war between our bodies' natural inclinations to rest and a host of competing factors created by human society.

Natural sleep patterns are guided by circadian rhythms that are set and reset by the natural cycle of day and night, adjusted by input from our eyes. Forger and Walch had used existing data from other studies to create their mathematical model that simulates these natural circadian rhythms.

This model also enabled them to explore the patterns that appeared during analysis of the ENTRAIN sleep data. For example, they found that people who spend time outdoors in natural light tend to go to bed earlier, and get more sleep, than those who spend most of their day in artificial light. But those data don't reveal if the light itself is causing more sleep, Walch says. For example, these people may report sleeping more because they have physical jobs, which keep them outdoors and tire them out. The model provided a way to test the impacts of outdoor light alone, and its results suggest that natural light does make people sleep more regardless of what they do while outside.

The results also led Forger to suggest an interesting hypothesis about how the battle between social influence and circadian rhythms plays out each day: “We noticed that when people wake up was not a good predictor of whether people in a certain country would sleep more or less, but when they go to bed really was,” he says. “So the reason why people are getting less sleep in certain countries is that they are going to bed later, rather than waking up earlier than people in other countries.”

That finding suggests to Forger that bedtime may be pushed back by social influences, such as working late or going out with others, but that wake time remains strongly guided by biological factors.

“I'd assumed wake time would really be a function of societal effects like alarm clocks,” he says. “But our data support the hypothesis that our biological clocks are governing when we wake up. For instance, we found that people in countries that have a later sunrise sleep in more.” The timing of sunset, meanwhile, may affect the total amount of sleep a person gets.

That hypothesis, however, is at odds with the results of other studies. “All our data and that of other people speak against this, and 85 percent of alarm clock users also demonstrate the opposite,” says Till Roenneberg, a professor at the University of Munich’s Institute of Medical Psychology. Roenneberg's work, in fact, suggests that your alarm clock may be hazardous to your health.

“There are, in my view, no easy answers to scheduling our 24/7 existence, [but] it is painfully clear we are currently damaging the lives of most people at the moment, and more immediate actions are required,” Kelley says. “The fundamental point is that there is wide variation in our individual [biological] timings over 24 hours. [It's] not a one size fits all phenomena.”

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