People who work shifts over the long term may have poorer brain power, the results of a new study suggest.

According to European researchers, it is already known that shift work disrupts the body's internal clock and this has been linked to a number of health problems such as heart disease and some cancers.

However until now, little has been known about the potential effect on the functioning of the brain. To investigate this further, the researchers monitored the cognitive abilities of over 3,000 adults at three different points in time - 1996, 2001 and 2006. During this time, the participants worked in a number of different sectors or they retired.

All underwent a number of cognitive tests to assess things such as short-term memory, long-term memory and processing speeds.

When the researchers looked at whether abnormal working hours were linked to a decline in cognitive abilities, they found that people who worked shifts, or had previously worked shifts, scored lower on memory, processing speed and overall brain power, compared to those who had never worked shifts.

The researchers then looked at the effect of shifts that rotated between mornings, afternoons and nights. Almost one in five of those who still worked, and almost one in five of those who had retired, had worked these types of shift patterns.

The study found that people who had worked rotating shifts for 10 years or more had lower overall cognitive and memory scores than those who had never worked such shifts.

In fact, their scores were equivalent to over six years of age-related cognitive decline.

The study also looked at whether stopping shift work led to any recovery in brain power. It found that it was possible to regain cognitive abilities if shift work was stopped, but this took at least five years.

"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 commented.

They added that their findings suggest that the health of people who have worked shifts for at least 10 years should be monitored.

Details of these findings are published in the journal, Occupational & Environmental Medicine.