An estimated 15 million Americans routinely work night shifts or rotate in and out of overnight shift work, and, as anyone who's ever been stuck working when others are sleeping can tell you, the odd hours can take a toll.

A study published this month in the Journal of Clinical Sleep Medicine suggests that it's insomnia, rather than sleepiness, that has the largest impact on night shift worker productivity.

The study looked at several dozen permanent night workers, the majority of whom were diagnosed with "shift work disorder" - a condition whose symptoms include excessive sleepiness at times when the worker is supposed to be awake, alert and productive, as well as a lack of concentration and energy, irritability and chronic insomnia.

The findings, financed by the Center for Disease Control and Prevention's National Institute for Occupational Safety and Health (NIOSH), found those classified as "alert insomniacs" were the most impaired when it came to work productivity and cognitive function. In other words, sleepless overnight workers who may seem more on-the-ball than their sleepy counterparts might in reality be the ones having the most trouble staying on top of their work.

"Our findings are important to everyone who is dealing with night shift work," principal investigator Valentina Gumenyuk, director of the MEG Neuroimaging Center at Meadowlands Hospital in Secaucus, New Jersey, said in a statement to the American Academy of Sleep Medicine.

"Our study reaffirms that insomnia within shift work disorder demands clinical attention," she added, "and it suggests that treatments focusing on the relief of excessive sleepiness in shift work disorder may not sufficiently improve work-related outcomes."

And management should pay attention, since blunders by impaired overnight workers can end up having serious consequences not only on productivity, but also on workplace safety and occupational health.

Researchers have been looking at ways to shift the body's circadian clock so that it can become more comfortable with working at night while sleeping during the day, but such adaptations are still far from easy. "The circadian clock is very stubborn and hard to push around," Charmane Eastman, a physiological psychologist at Rush University in Chicago, told the American Psychological Association back in 2011.

Source:CBS News. Sleepless on the night shift? Not just a health issue. http://www.cbsnews.com/news/sleepless-on-the-night-shift-its-not-just-a-health-issue/