Personal Finance

Memo to work martyrs: Long hours make you less productive

Research shows that sleep deprivation seems to cause a person to eat more.
Zero Creatives | Getty Images

Nearly half of U.S. workers say they routinely put in more than 50 hours on the job each week, often without overtime pay. But employers should probably start politely declining the "free" gift, new research suggests.

So-called "work martyrs" give hundreds of hours in free labor to their employers every year, encouraged by always-on gadgets, work through nights, weekends, and vacations. Trading sleep or fun for unpaid work is obviously a bad deal for employees, but there's a growing body of evidence that even apparently "free" labor might not be a good deal for employers, either.

Research that attempts to quantify the relationship between hours worked and productivity found that employee output falls sharply after a 50-hour work-week, and falls off a cliff after 55 hours—so much so that someone who puts in 70 hours produces nothing more with those extra 15 hours, according to a study published last year by John Pencavel of Stanford University. Longer hours have also been connected to absenteeism and employee turnover. The Centers for Disease Control and Prevention even has an entire website devoted to the effects of long working hours even if workers aren't paid for this extra time. It's not free, Pencavel points out.

Read MoreYou won't believe the perks some companies are offering

"There are ancillary costs of long working hours such as the expenses of running complementary machinery and of providing light, heat, ventilation, and supervisory labor," he said in the study.

The idea that work hours can be cut without a drop in productivity should be good news for both American workers and their bosses, who routinely put in more hours than seem productive. In a Gallup poll last year, 4 in 10 Americans said they work more than 50 hours every week, and 2 in 10 more than 60 hours. The average work week was 47 hours.

Despite the research, don't expect Americans to be better about getting home for dinner any time soon. Not only are hours worked per week on the rise, but technology seems to be irresistibly driving the trend. A recent Pew survey found that 35 percent of adults say the Internet, email and cellphones have increased their hours worked. For office workers, the number rises to 47 percent.

Also contributing to this nonvirtuous cycle of overwork: Employees are trading sleep for work. Researchers at the University of Pennsylvania Perelman School of Medicine examined detailed time-use studies and found that people who said they slept six hours or less at night worked 1.5 hours more than others who got more sleep.

CEO shares tips on surviving little sleep
CEO shares tips on surviving little sleep

"The evidence that time spent working was the most prominent sleep thief was overwhelming," lead study author Mathias Basner said when the results were released last month. "It was evident across all sociodemographic strata and no matter how we approached the question."

Sleep-deprivation is also a productivity killer, but Pencavel's research more directly addresses productivity declines as a function of hours worked. He used a unique dataset from a World War I munitions factory, where productivity was easy to measure and the need for output was infinite. Pencavel's real contribution comes in spotting the productivity cliff. Other research has demonstrated more hours don't necessarily equal more results, but his research suggests a natural limit to work days and work weeks. Pencavel calls it a "highly nonlinear effect."

"At 35 hours, an additional five hours to the length of the working week has consequences for the effective labor input that are quite different from an additional five hours starting at 48 hours," he writes.

Of course, working at a munitions factory isn't the same as working in an office—for starters, productivity measures are far more elusive for knowledge workers—but Basner's research builds on plenty of other evidence about the futility of overwork. While short bursts of intense work with overtime can be productive, and aren't always harmful, in the long run, excessive hours are counterproductive.

Read MoreAre you leaving money on the table at work?

"The simple reality is that work, both mental and physical, results in fatigue that limits the cognitive and bodily resources people have to put towards their work," said Ken Matos, senior director of research at the Families and Work Institute think tank. "When they are not thinking clearly or moving as quickly or precisely they must work more slowly to maintain quality and safety requirements."

It's not fair to blame overwork exclusively on employers or middle managers. Plenty of workers bring it on themselves. In her book "What the Most Successful People Do on the Weekend," Laura Vanderkam argues that many workers simply lack the self-discipline to set proper boundaries between work and home, and to finish all the nonemergency work tasks between 9 a.m. and 5 p.m. Others thrive on the sense of self-importance they feel from working late or on weekends.

"You have to set an appointment to get off the grid as surely as to go on it," she wrote.

And employers are hardly ignoring the issue. Work-life balance programs are in vogue, and there are celebrated cases of executives—like Facebook's Sheryl Sandbergbeing very public about their efforts to be home for dinner, notes Russell Clayton, a management professor at Saint Leo University. Many firms fail, however, because the efforts are largely symbolic, he said.

Read More 7.2 million Americans hiding money from spouses

"There's oftentimes a disconnect between the company mantra and the standards supervisors are holding their employees to," he said. "For example, Company XYZ's top exec comes out and says that he or she makes it a point to leave each workday by 5:30 pm and believes each of his/her employees should do the same. ... But if my immediate supervisor at Company XYZ doesn't see value in that, then it is unlikely that I'll have the freedom to routinely leave at a reasonable hour. It's similar to the way companies will have 'family-friendly' policies such as flextime or work-from-home. The policy is there, but if your direct supervisor doesn't allow you to use it ... it's useless."

At least as useless as those unpaid overtime hours workers put in after falling off the productivity cliff.