Working shifts rather than traditional office hours was found to impair memory and thought processing over time.
Researchers examining the impact of shift work have found that long-term shift work - for 10 or more years - had a strong negative impact on the health of the brain, and although the effects could be reversed, full recovery could take as long as 5 years. Their work has been published online in Occupational & Environmental Medicine.
In a similar way to chronic jet lag, shift work is known to disrupt the body's natural time-keeping system, referred to as the circadian rhythms.
Circadian rhythms influence body temperature, sleep and wakefulness and various hormonal changes, helping the body to function to its full potential according to what time in the cycle (usually the length of a day) the brain thinks it is.
Disruption to circadian rhythms has been linked to a range of health problems, including breast cancer, cardiovascular disease, reproductive difficulties and ulcers. It has also been associated with acute effects on safety and productivity at work. However, until now, little has been understood about how it could impact on the functioning of the brain.
Observing the effects of shift work
For the study, a team of researchers analyzed a sample of over 3,000 people, taken from the patient lists of three occupational health doctors from different regions in southern France.
The cognitive abilities of the patients were tracked in 1996, 2001 and 2006, using tests designed to assess long- and short-term memory, processing speed and overall cognitive ability. A total of 1,197 of the participants were assessed at all three points in time.
Participants were aged 32, 42, 52 or 62 at the time of the first tests. They were either working or retired, with 1,484 people reporting working shifts for at least 50 days of the year. Around 1 in 5 of those in work (18.5%) and those who had retired (17.9%) reported working a shift pattern that rotated between morning, afternoon and night shifts.
The researchers initially looked to see whether any amount of shift work - referred to as "non-standard working hours" - was linked to a decline in cognitive abilities. They found that participants who were currently or who had previously worked in shifts scored lower than those working normal office hours in the tests assessing memory, processing speed and overall brain power.
Participants who had worked with a rotating shift pattern for 10 or more years were found to have much lower overall cognitive and memory scores than those who had never worked in that manner. The lower scores were found to be equivalent to 6.5 years of age-related cognitive decline.
Analyses were also made to determine whether stopping shift work was associated with an improvement in cognitive abilities following impairment. Results suggested that recovery was possible, although this was found to take at least 5 years.
Potential safety consequences
"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," write the researchers, "given the increasing number of jobs in high hazard situations that are performed at night."
Although their study is an observational one and so no complete conclusions can be drawn regarding causation, the researchers believe that the manner in which shift work can disrupt the circadian rhythms could generate psychological stressors, impairing the functioning of the brain.
The authors of the study also note that other research has previously suggested shift workers could be at an increased risk of vitamin D deficiency as a result of their reduced exposure to daylight. In the past, vitamin D deficiency has also been associated with the impairment of cognitive functioning.
"Measures should be considered that mitigate the impact that prolonged exposure to shift work has on cognitive abilities, including switching to normal day work," suggest the researchers.
Earlier in the year, Medical News Today reported on a study exploring how the disruption of circadian rhythms could contribute to inflammatory bowel disease and other similar conditions.