A study has found that working anti-social hours can prematurely age the brain and dull intellectual ability.

The research, by an international team at the University of Toulouse in France, found that this type of work led to an impaired ability to think and remember.

A decade or more working rotating shifts was associated with a loss of brain function equivalent to 6.5 years of age-related cognitive decline, the research showed.

Stopping shift work led to gradual recovery, but one that took at least five years, said scientists.

Disruption of the body clock, which is based on natural day and night cycles, may cause stresses that may affect brain functioning, the researchers believe.

Other studies have linked vitamin D deficiency due to reduced exposure to sunlight to poorer mental ability.

Writing in the journal Occupational and Environmental Medicine, the international team led by Dr Jean-Claude Marquie, concluded: "Shift work chronically impairs cognition, with potentially important safety consequences not only for the individuals concerned, but also for society."

The scientists assessed more than 3,000 workers from southern France who had their mental abilities tested on three occasions over a ten-year period.

Participants were aged 32, 42, 52 and 62 at the time of the first test in 1996.

Around a fifth had worked a shift pattern that switched between mornings, afternoons and nights.

Shift workers had lower average scores for memory, processing speed and overall brain function than those working normal office hours.

Compared with people who had never worked rotating shifts, participants employed this way for ten or more years had lower overall thinking and memory scores.

The scientists, who included researchers from the University of Swansea, said the problems increased with the length of time people worked shifts.

After ten years of rotating shift work the association became "highly significant".

However, there was evidence that the deficits were reversible.

People who stopped working shifts recovered their lost mental function after at least five years.

The researchers wrote: "Measures should be considered that mitigate the impact that prolonged exposure to shift work has on cognitive abilities, including switching to normal day work."