Working non-standard hours -- often called "shift work" -- for many years is not only hard on the body, but may also dull the mind, new research suggests.

According to the study, those who do shift work for more than 10 years seem to have the equivalent of an extra 6.5 years of age-related decline in memory and thinking skills.

This study, however, only found an association between shift work and impairments in memory and thinking (sometimes called "cognition"). It didn't prove that shift work was the definitive cause of those changes.

Effect persists after shift-work ends

"Our work suggests that shift work is associated with impaired cognition, that the association is stronger and especially significant for exposure durations exceeding 10 years, and finally, and maybe most importantly, that the effect persists after having ceased any form of shift-work schedule," said Jean-Claude Marquie, research director at the National Center for Scientific Research, at the University of Toulouse, France.

Recovery of thinking skills took at least five years, his study found.

Past research has shown that shift work can affect alertness and thinking and memory. Fewer studies have looked at whether there is a chronic impact, according to background information in the new report.

How the study was done

To get a better idea of the possible long-term effects, Marquie and colleagues tracked the mental abilities of more than 3,000 people from different regions in France who were employed in a wide range of sectors or who had retired. The researchers evaluated the study participants in 1996, 2001 and 2006.

The men and women were aged 32, 42, 52 and 62 when they took the first set of tests to gauge memory, processing speed and overall thinking ability. About half of the study participants had done shift work at least 50 days of the year.

More than 1,000 current and retired employees worked a rotating shift pattern that switched back and forth among morning, afternoons and nights, according to the study.

Overall, shift workers had lower memory, thinking and processing speed scores than those who had worked only standard hours, the investigators found.

Those who worked a rotating shift had lower overall memory and thinking ability scores compared to those who never did shift work. Those who had done rotating shift work for 10 years or more had even lower scores, the study revealed.

Not dramatic, but evident

The differences weren't dramatic, but they were evident, the authors noted. For instance, on one scale measuring thinking, memory and speed called the global cognitive performance score, those who never did shift work scored 56 points out of a possible 100, while those who worked rotating shifts for more than 10 years scored about 52, according to the results.

It took at least five years for workers to recover mental skills, except for processing speed, the researchers found.

Marquie can't explain for sure why the shift work, especially rotating shift work, is linked with an impact on thinking skills. However, he said, it might be tied to the stress generated by disruption of the body's internal clock (circadian rhythms). Changes in the circadian rhythm can increase levels of the stress hormone cortisol. This can have an adverse effect on the brain, especially areas of the brain important for memory, Marquie said.

The findings, published online Nov. 3 in Occupational and Environmental Medicine, highlight the importance of monitoring the health of shift workers, Marquie added, especially those working shift patterns for longer than 10 years.

Christopher Colwell, professor of psychiatry and director of the laboratory for circadian and sleep medicine at University of California, Los Angeles, David Geffen School of Medicine, reviewed the study's findings.

Not a new finding

"It's not a new finding, it's something that all of us intuitively know," he said, "but it's a well-documented finding."

Colwell agreed that researchers don't know the exact mechanism of why shift work has such an effect. However, he said, it is believed that "when you desensitize the rhythm inside your body, the circadian rhythm, as a consequence you have memory deficits."

That would show up in lab tests, he said, such as when you are asked to press a button when a light comes on, for instance.

Sleep is important for memory, Colwell said. "What we can see very clearly, especially in animal models, is that one of the things that seems to occur during sleep is that the brain stores information into long-term memory." When sleep is disrupted, he said, so is the storage of that information.

Some people adapt better to shift work than others, he noted. "Those who live their lives in reverse do better, in general," Colwell said. In other words, they keep their schedule exactly opposite of those who work standard hours.

Take a nap

Colwell said shift workers shouldn't "freak out" at the findings, but be aware. One countermeasure to make up for lack of sleep? Taking a nap can help, he said, but keep it to 45 minutes or less for the best benefits.