A growing number of businesses are encouraging their employees to work when their bodies are most awake.

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A few years ago, scientists conducted a real-world experiment at a ThyssenKrupp steel factory in Germany. They assigned the day shift to early risers and the late shift to night owls.

Soon the steel workers, many of whom had been skeptical at the outset, were getting an extra hour of sleep on work nights. By simply aligning work schedules with people’s internal clocks, the researchers had helped people get more and better rest.

“They got 16 percent more sleep, almost a full night’s length over the course of the week,” said Till Roenneberg, a chronobiologist at Ludwig-Maximilian University in Munich, who headed the study. “That is enormous.”

In recent years, U.S. educators have been paying increased attention to their students’ sleep needs, with growing debate about delaying school start times. Now a number of businesses are following suit, encouraging their employees to work when their bodies are most awake.

“It’s a huge financial burden not to sleep properly,” Roenneberg said. “The estimates go toward 1 percent of gross national product,” both in the United States and Germany.

Emerging science reveals that each of us has an optimal time to fall asleep and wake up, a personalized biological rhythm known as a “chronotype.” When you don’t sleep at the time your body wants to sleep — your biological night — you don’t sleep as well or as long, setting the stage not only for fatigue, poor work performance and errors but also health problems ranging from heart disease and obesity to anxiety and depression.

A full 80 percent of people have work schedules that clash with their internal clocks, said Céline Vetter, an assistant professor at the University of Colorado at Boulder and director of the university’s circadian and sleep epidemiology lab. “The problem is huge,” Vetter said. “If we consider your individual chronotype and your work hours, the chances are very high that there’s quite a bit of misalignment.”

Put it this way: If you rely on an alarm clock to wake up, you’re out of sync with your own biology.

Studies on workers in the call center of a mobile phone company, a packaging manufacturer and an oil transportation company show that these employees are more stressed and may experience more work-related discomfort and pain. It’s the mismatch — not the hours themselves — that matters. A 2015 Harvard Medical School study found that for night owls, working during the day increases diabetes risk.

Among the companies seeking to remedy the problem is Southwest Airlines, which allows pilots to choose between morning and evening flight schedules. The U.S. Navy recently traded an 18-hour submarine shift schedule for a 24-hour one that more closely matches sailors’ biological rhythms. And at some pharmaceutical, software and financial companies, managers expect employees to come to the office for only a few hours in the middle of the day — or to work off-site entirely.

“I think circadian rhythms will be a huge issue for human resources in the future,” said Camilla Kring, a Danish consultant who has helped employees at AbbVie, Roche, Medtronic and other companies learn to respect their natural sleep cycles. “It really makes sense to think about when people have the most energy and when they’re peaking mentally.”

Worker fatigue has played a role in many workplace accidents, most famously the Challenger space shuttle explosion and the Exxon Valdez oil spill, but no doubt countless more on the commute to and from work. The National Highway Traffic Safety Administration estimates that drowsy drivers cause 16.5 percent of fatal crashes.

A survey released last year by the National Safety Council found that 97 percent of workers have at least one risk factor for fatigue, with 27 percent reporting having unintentionally fallen asleep on the job in the previous month and 16 percent reporting they had experienced at least one safety incident because of fatigue.

“We have a 24/7 society, so we knew fatigue was definitely affecting part of our workforce,” said Emily Whitcomb, senior program manager of the council’s fatigue initiative. “But we did not anticipate that fatigue was going to be affecting almost 100 percent of our workforce.”

Sleep deprivation may even lead to ethical breaches at work. In a 2014 experiment, Christopher Barnes of the University of Washington and colleagues asked 142 people with extreme chronotypes to show up at their laboratory either at 7 a.m. or at midnight for a dice-rolling game that offered a $500 prize. Reporting was on the honor system. Morning people lied 18 percent more when they did the task at midnight than when they did it in the morning, whereas night owls lied 16 percent more when doing the task at 7 a.m.

Roenneberg has collected data from 300,000 people and found that chronotypes plot as a bell-shaped curve, with a few individuals at each extreme and most falling somewhere in the middle. According to Roenneberg’s research, the most frequent chronotype — held by about 13 percent of the population — sleeps from around midnight to 8 a.m. Thirty-one percent of people have an earlier natural bedtime, and 56 percent have a later one. That means for at least 69 percent of the population, getting to the office by 8 or 9 a.m. requires waking up before their body is ready.

Not all experts acknowledge such fine-grained distinctions, instead grouping people into morning, evening and “intermediate” types. Regardless, while true “larks” and “owls” tend to dominate the conversation, they make up a small percentage of the population.

To determine your chronotype, imagine that you have two weeks of vacation to spend as you like, with no evening or morning commitments and no pets or children to wake you. Chronotypes reflect habits as well as biology, so you would also need to eliminate caffeine and avoid artificial light at night, which pushes a person’s chronotype later. At what time would you tend to fall asleep and wake up? Don’t be surprised if you’re unsure. After years spent accommodating work, family and social commitments at the expense of sleep, “a lot of people don’t know what rhythm they have,” Kring said.

Chronotypes shift in a predictable way over the course of a lifetime.

Between the ages of 12 and 21, everyone’s natural sleep schedule gets about 2 1/2 hours later — which is why adolescents have so much trouble waking up for school. After that, chronotype creeps in the other direction, which is why older people typically find themselves waking earlier than they used to. But chronotype determines more than when you sleep and wake. It orchestrates predictable peaks and troughs of energy over the course of the 24-hour day.

The “window of circadian low,” the hours when the body is least adapted for wakefulness, typically occurs between 2 a.m. and 6 a.m. There’s another, smaller dip 12 hours later, in the midafternoon.

There are also two high points, when thinking is sharp and reaction times quick. One occurs within an hour or two after waking, and the other after the daytime dip. This cycle is shifted earlier in a morning person and later in an evening or night person.

At the Denmark offices of pharmaceutical company AbbVie, employees design work schedules that take advantage of their biological strengths. A nine-hour training program helps them identify when they are ripe for creative or challenging projects, typically mornings for early risers and afternoons for late risers. Lower-energy periods are meant for more mundane tasks, like handling emails or doing administrative chores. Workers save commuting time by avoiding rush hour traffic, and can better mesh their personal and professional lives — for example, by getting their children from school in the afternoon, then working from home at night after the children are in bed.

According to company surveys, employee satisfaction with work-life balance has risen to nearly 100 percent today, from 39 percent 10 years ago, when the program started.

Last year, the Denmark division of Great Place to Work, a global organization that ranks companies based on employee satisfaction, named AbbVie the top middle-size company in the country. “The flexibility actually empowers people to deliver the best possible results,” said Christina Jeppesen, the company’s general manager.

A 2018 survey by the Society for Human Resource Management, which represents 300,000 HR professionals in more than 165 countries, found that 57 percent of its members offer flexible hours, 5 percent more than in 2014. “Managers who give it a try often find that employees’ morale, engagement and productivity all go up, because they are working at a time that works best for them, and able to get the most work done,” said Lisa Horn, the group’s vice president for congressional affairs.

Some companies restrict meetings to “core hours,” between, say, 10 a.m. and 3 p.m., to accommodate various schedules. Others build flextime into the workweek. At Phase 2, a software development firm in Oklahoma City, each week ends with “productivity Friday,” when employees are expected to work remotely in a place of their choice, whether that be from home, a coffee shop or at a weekend house at the lake.

Stefan Volk, a senior lecturer at the University of Sydney Business School, has suggested that businesses can leverage chronotypes to maximize team success. For example, members of a surgery team should have similar chronotypes because they need to be in top form simultaneously. But at a nuclear power plant, workers should have different energy peaks, so that someone is always on the alert.

But while lots of corporations promise flexibility, veering from the traditional 9 to 5 work hours requires a cultural shift. A 2014 study led by Barnes found that many managers have an ingrained prejudice in favor of early birds, whom they perceived as more conscientious simply because they arrived at work early, a view that could dissuade some workers from using flextime.

But sticking to traditional hours can be counterproductive, leading to “presenteeism” — employees showing up and being only minimally functional. “Companies are wasting the potential of their people,” Volk said. “You have someone sitting there from 7 til 9 a.m. sipping coffee, being completely unproductive, and then you send them home at 4 when they actually start getting productive.”

For many office workers, the answer may be as simple as delaying work start times an hour or two — say until 9:30 or 10 a.m. Since many people are in the middle of the chronotype continuum and wake naturally around 8 or 9 a.m., such a modest shift could provide widespread relief. “We’re talking about one hour,” Kring said, “not a revolution.”