A new study has said that people who work in shifts for 10 years or more may suffer loss of memory and brain power. The study also warned of safety concerns in high-risk jobs.
The effects on brain function can be reversed, the team wrote in the journal Occupational & Environmental Medicine, but this may take at least five years.
The research is the latest to highlight the dangers of shift work, which disrupts the body's internal clock and has previously been linked to health problems like ulcers, cardiovascular disease and some cancers.
Yet, little has been known about its potential impact on brain function.
Researchers tested more than 3,000 current or retired workers in a variety of sectors in southern France in 1996, 2001 and 2006 for long- and short-term memory, processing speed and overall cognitive abilities.
About half of the trial subjects, aged either 32, 42, 52 or 62 when they were first tested, had worked shifts -- classified as night work or shifts that alternated between morning, afternoon and night.
Comparing the change in test results over time, and between the two groups, the researchers found an association between shift work and "chronic cognitive impairment".
"The association was stronger for exposure durations exceeding 10 years" of shift work, which they said was equivalent to an additional 6.5 years of age-related decline.
The data also showed that "recovery of cognitive functioning after having left shift work took at least five years."
The study could not prove conclusively that shift work was the cause of the cognitive decline, said the authors, and though it seemed "highly plausible", further research was needed.
The findings raised "potentially important safety consequences not only for the individuals concerned, but also for society," the scientists concluded, pointing to "the increasing number of jobs in high hazard situations that are performed at night".
"The current findings highlight the importance of maintaining a medical surveillance of shift workers, especially of those who have remained in shift work for 10 years or more."