"The cognitive impairment observed in the present study may have important safety consequences not only for the individuals concerned, but also for society as a whole, given the increasing number of jobs in high hazard situations that are performed at night," the researchers warned.

However, the study led by Jean-Claude Marquie of the University of Toulouse in France found that once people stop working in shifts, it is possible for them to recover the memory loss. But it could take five long years.

The researchers tracked the cognitive abilities of more than 3,000 people who were either working, in a wide range of sectors, or who had retired, at three time points: 1996; 2001; and 2006.

The data showed that those who currently, or who had previously, worked shifts had lower scores on memory, processing speed, and overall brain power than those who had only worked normal office hours.

The second set of analysis looked at the impact of working a rotating shift pattern and found that compared with those who had never worked this type of shift, those who had, and had done so for 10 or more years, had lower cognitive and memory scores.

The study appeared in the British Medical Journal.

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