Can you suffer the symptoms of jet lag without even stepping foot outside your current time zone? Much to the chagrin of the reluctant traveler, the answer is apparently yes.
And, if you’re reading this, you probably already have. A hefty chunk of the human population—especially those living in particularly plugged-in parts of the world—maintains a different sleep schedule on weekdays than on weekends, staying up later and sleeping in on their days off from work.
“This study is indicative of the amazing amount of data that’s out there,” says Martha Merrow, who studies the biology of sleep at the Ludwig Maximilian University of Munich’s Institute of Medical Physiology, but did not participate in the research. “It shows the power of social media in revealing aspects of human behavior to us.”
When left to their own devices, our bodies’ internal clocks naturally respond to sunlight, the evolutionarily-ancient cue of the day’s beginning and end. But in the last few centuries, humans have retreated indoors for both business and pleasure, diminishing our exposure to the outside environment and gradually uncoupling our physiologies from the Sun. Now, rather than being governed by available daylight, our schedules are shackled to the constantly changing demands of society. And as a result, there’s a serious mismatch between the times we’re asleep on weekdays versus weekends.
“There’s this persistent idea that light, because it’s not tangible, doesn’t have an effect on your body,” says study author Olivia Walch, a mathematician and sleep researcher at the University of Michigan. “And that’s not true. Light is this huge factor in sleep that doesn’t get the credit it deserves.”
Social jet lag tends to go hand-in-hand with chronic sleep-deprivation, which itself is a public health issue that costs the United States up to $411 billion—over 2 percent of our gross domestic product—each year. But for most individuals, social jet lag also entails being active when one should be asleep and vice versa, putting people in all-too-frequent states of physiological disorientation. The effects of social jet lag spill over to many other aspects of health and wellbeing: Toiling through this weekly flux has been tied to heavier drinking and smoking, as well as obesity, depression, and even decreased academic performance.
Not everyone experiences social jet lag in the same way. On the one hand, each person’s internal clock is unique. Some people are early risers, while others are night owls, for example—and it’s often the latter group that struggles the most transitioning between the slog of early weekday mornings and their more natural nocturnal pattern on weekends. Even the duration of someone’s morning commute or the latitude of one’s hometown can affect the ways in which an individual experiences, and thus suffers from, sleep schedule variations.
Gathering this kind of data can be tough through traditional mail-and-return surveys, especially considering the staggering diversity of culture, climate, and careers in the United States alone. So a team of researchers, led by biophysicist Eugene Leypunskiy and biologist Michael Rust of the University of Chicago, looked to the modern era’s best proxy for human behavior: the Internet—more specifically, the social media juggernaut of Twitter.
The new report, published today in the journal Current Biology, tracks every publicly available, geographically-tagged tweet from the beginning of 2012 to the end of 2013—two years of data from about 246,000 Twitter users in 1,521 counties across the entire United States. “We weren’t interested in looking at what people were tweeting, but when they were tweeting,” Rust explains.
The researchers found that, across the country, tweets—like many Americans—rose slowly in the morning, increased steadily to a peak in the afternoon or evening, then dipped back down at typical bedtimes. Lengthy lulls in Twitter activity indicated when people were probably asleep. And, as expected, these tweeting troughs shifted to later intervals on weekends relative to weekdays. Across counties, the average amount of “Twitter social jet lag”—or the amount of time people were shifting their sleep schedules between work and free days—clocked in at about 75 minutes.
When the researchers mapped tweeting patterns across time zones, they discovered striking differences based on geography. For one, latitude and longitude both had an effect on social media habits. In general, tweeters in the American Southwest took longer breaks at bedtime than their Central U.S. and Northeastern counterparts and had less social jet lag overall. The westernmost counties within time zones, however, experienced more pronounced social jetlag—which may have to do with the fact that the Sun rises later in local time in these regions.
Across counties, certain lifestyle habits also seemed to predict a higher degree of social jet lag, including earlier average start times for commutes and the proportion of the population involved in shift work—both of which may indicate less flexibility in weekday schedules. Pronounced Twitter social jet lag also coincided with higher obesity rates, mirroring previous findings.
Additionally, Twitter social jet lag ebbed and flowed with the seasons. The sleep schedule mismatch was most severe in early autumn and late winter, but mellowed out during the summer. When the researchers examined the relationship closer, they found that a common contributor to this pattern was the occurrence of summer breaks from school, during which some families seemed shift their weekday sleep cycles later to more closely match their (likely more biologically natural) weekend schedules. The same rules appeared to apply to school holidays spiked intermittently throughout the rest of the year.
“The amount of social jet lag can be thought of as a proxy for how on vacation these places are,” Walch explains.
Not all counties experienced reprieve from social jet lag in the summer, however. Certain locales felt the same amount of mismatch year-round—and these counties also tended to be more urban, and host residents that attended college, commuted late in the day, and were less likely to smoke or be obese.
Because counties vary in not only their physical location, but also countless cultural characteristics, Rust says, it’s still difficult to disentangle exactly what factors contribute the most to social jet lag place by place. “We were looking at 1500 counties in the US,” Leypunskiy adds, “and every single one tells its own story.”
But Merrow suggests it’s likely that culture plays a role. For instance, the tweeters of Orange County, California appear to have longer nighttime lulls in Twitter activity and milder social jet lag than Lafayette County, Louisiana—but the two counties lie at similar latitudes, indicating that the drivers behind behavior may be more complicated than sunlight exposure.
Fortunately, there may still be a silver lining to the plight of social jet lag. Till Roenneberg, a sleep biologist at the Ludwig Maximilian University of Munich’s Institute of Medical Physiology, advises trying to tailor our schedules to our natural rhythms—which inevitably differ from person to person. Night owls, for instance, may benefit from nudging their work schedules later—a strategy that could help close the gap between weekday and weekend sleep habits without compromising total rest. But of course, not everyone has this flexibility. According to Roenneberg, society has yet to wake up to the idea of respecting sleep.
An easier fix? Simply being a little more aware of our exposure to light at both appropriate and inappropriate times, Walch says. If your goal is to fall asleep earlier at night, you might try minimizing evening phone time (or, at the very least, use an app that will filter out the stuff most likely to mess with our internal clocks: short wavelength blue light). Or take it a step further and avoid bright electronics entirely in the hours before bedtime—even if it means forsaking that final midnight tweet.
“Social jet lag is something people experience all the time and don’t even realize it,” Leypunskiy says. “And I think if people want to change it, they have the means to do it… but it’s probably not a one-size-fits-all thing.”
In the future, Rust hopes to zoom in on the biology of sleep at an individual level. Aggregations of tweets can only tell us about the average activity of a whole county, he explains. But gathering information from something like a Fitbit or other personal health monitor—with appropriate consent—could shed light on each person’s behaviors, patterns, and preferences.
Even a future analysis of the content of tweets could be informative about the culture of social jet lag, says Aron Culotta, a computer scientist who conducts social media analysis at the Illinois Institute of Technology. “That’s one huge advantage of social media,” he explains. “You get to explore the ‘why’ behind a behavior.”
However, Culotta cautions that, as with any work that draws from social media, findings from a subset of the population may not always capture the entire picture. Not everyone’s on Twitter—and there could be bias in the way people tweet on weekdays versus weekends. Nonetheless, he praised the current study for its “rigorous approach” to social jet lag research.
For Rust, this study has already showcased the astounding diversity of humankind. “Many of us live in bubbles and see people who live similar lives to us,” he says. “But the way people live their lives and structure their time is very different—it isn’t like there’s a generic American.”