Working abnormal hours in the long-term may lead to memory loss and slower, irreversible brain processing speed, suggests a new study published in British Medical Journal.

Researchers in Sweden and France tracked the cognitive abilities of more than 3,000 people who were either working in a range of sectors or who had retired at three points: 1996, 2001 and 2006, according to a news release.

About half of the sample was drawn from a pool of patients of three occupational health doctors in southern France, who had worked at least 50 days of the year.

Participants were 32, 42, 52 and 62 years of age during the first set of tests, which assessed long- and short-term memory, processing speed and overall cognitive abilities.

About one in five of those working and a similar proportion of those who had retired had worked shifts that rotated among mornings, afternoons and nights.

Researchers found that those participants who currently or had previously worked abnormal shifts had lower scores on memory, processing speed and overall brain power than those who had worked the same hours every day.

The study authors also found that compared with people who had never worked in shifts, those who had done so for 10 or more years had lower global cognitive and memory scores— which equated to 6.5 years of age-related cognitive decline, the news release noted.

But researchers found that it was possible to regain cognitive abilities five years after ceasing shift work. The negative impact on processing speeds, on the other hand, was not reversed after the participants stopped working irregular hours.

Researchers noted that the study was observational, so while there was a link between shift work and decreased cognitive abilities, the abnormal hours didn’t definitively cause the decline in brain power.

They predicted that the disruption of the body clock as a result of shift work may have increased physiological stressors, which could have weakened brain function.

Experts have compared shift work to chronic jet lag because the two habits disrupt the body’s internal clock, or circadian rhythms. Previous studies indicate disrupting these rhythms can lead to ulcers, cardiovascular disease, metabolic syndrome and some cancers.

"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 wrote in their paper.