"Considerable evidence shows that overwork is not just neutral — it hurts us and the companies we work for. Numerous studies by Marianna Virtanen of the Finnish Institute of Occupational Health and her colleagues (as well as other studies) have found that overwork and the resulting stress can lead to all sorts of health problems, including impaired sleep, depression, heavy drinking, diabetes, impaired memory, and heart disease. Of course, those are bad on their own. But they’re also terrible for a company’s bottom line, showing up as absenteeism, turnover, and rising health insurance costs. 

"Even the Scroogiest of employers, who cared nothing for his employees’ well-being, should find strong evidence here that there are real, balance-sheet costs incurred when employees log crazy hours.

"If your job relies on interpersonal communication, making judgment calls, reading other people’s faces, or managing your own emotional reactions — pretty much all things that the modern office requires — I have more bad news. Researchers have found that overwork (and its accompanying stress and exhaustion) can make all of these things more difficult.

"Even if you enjoy your job and work long hours voluntarily, you’re simply more likely to make mistakes when you’re tired — and most of us tire more easily than we think we do. Only 1-3% of the population can sleep five or six hours a night without suffering some performance drop-off. Moreover, for every 100 people who think they’re a member of this sleepless elite, only five actually are. The research on the performance-destroying effects of sleeplessness alone should make everyone see the folly of the all-nighter.

"Work too hard and you also lose sight of the bigger picture. Research has suggested that as we burn out, we have a greater tendency to get lost in the weeds.

"In sum, the story of overwork is literally a story of diminishing returns: keep overworking, and you’ll progressively work more stupidly on tasks that are increasingly meaningless."

- Sara Green Carmichael, The Research is Clear: Long Hours Backfire for People and Companies, Harvard Business Review