Tuesday November 4 2014
Shift work has been linked to a range of conditions
“Shift work dulls your brain,” BBC News reports. In a French study, researchers assessed 3,232 adults using a variety of cognitive tests and compared the results between people who reported they had never performed shift work for more than 50 days per year with those that had. They analysed the results, comparing the number of years of rotating shift work performed and how long ago the shift work had stopped.
They estimated that working shifts for 10 years or more "aged" the brain by 6.5 years. They also estimated that it takes at least five years of non-shift working to reverse the effects, though this was not based on individuals' recovery of cognitive abilities. It was based on a snapshot comparing people who had stopped shift work more than five years before with people who were currently doing shift work or had never done shift work.
The study did not prove shift work causes cognitive decline, as it did not take into account people’s baseline cognitive ability.
It is also unknown whether the small, observed differences in cognitive performance scores would have had any meaningful difference in terms of daily life and functioning.
So if you are reading this on a break during your night shift, you should not be overly concerned.
Where did the story come from?
The study was carried out by researchers from the University of Toulouse, Swansea University, Stockholm University, the Université Paris Descartes and the University of Monaco. It was funded by several French national organisations and the UK Institute of Occupational Safety and Health.
The study was published in the peer-reviewed medical journal Occupational and Environmental Medicine.
The UK media reported the findings accurately. However, what was not made clear in reports was that, although the participants were assessed on three occasions, the analysis of recovery was based on only one time point. Therefore, this does not prove that an individual will recover their cognitive abilities after stopping shift work. Media reports also did not made it clear that the differences seen could have been due to natural abilities, rather than shift work.
What kind of research was this?
This was a cohort study that aimed to assess the impact of shift work on mental ability. As it was a cohort study, it is useful to look for associations; however, it cannot prove causality as it does not take all other factors into account.
What did the research involve?
In 1996, 3,232 adults aged 32, 42, 52 or 62 years old were randomly recruited from French registries of salaried or retired workers. They completed questionnaires, had a clinical examination and performed a variety of well-validated cognitive tests, such as being asked to read 16 words three times and then immediately reciting the list from memory.
The results of these tests were pooled to provide a score for global cognitive performance, memory and processing speed on a scale of 0 to 100, with 100 indicating a higher performance. They were invited to have similar tests five and 10 years later. A total of 1,197 people attended on all three occasions.
The participants were also asked if their work involved any of the following types of shift work for more than 50 days per year, with responses categorised as either "current", "past" or "never":
- rotating shift work (for example, alternating morning, afternoon and night shifts)
- schedules that did not allow them to go to bed before midnight
- work requiring them to get up before 5am
- work preventing them sleeping during the night (night work)
The researchers also calculated the amount of exposure to rotating shift work and analysed whether longer duration of this type of shift work had any effect on the cognitive test scores. They grouped the participants according to:
- never worked rotating shifts
- 10 years or less
- more than 10 years
Finally, they analysed whether the scores differed between people who were currently doing rotating shift work or those who had stopped more or less than five years before and people who had never done shift work.
They performed statistical analyses to take into account the following confounders:
- socioeconomic position
- sleep problems
- perceived stress
- alcohol consumption
- tobacco consumption
What were the basic results?
At baseline, the 1,635 people who reported never having done shift work for more than 50 days per year had higher average global cognitive performance scores compared to 1,484 people who had experienced shift work (56.0 compared to 53.3). This difference remained the same at each time point in the study. They also had slightly better memory scores (50.8 versus 48.5) and speed processing scores (78.5 versus 76.5).
The global cognitive performance scores were highest in the group aged 32 (59.6) and lowest in the group aged 62 (47.7).
People with more than 10 years of exposure to rotating shift work had poorer cognitive scores compared to those who had never worked rotating shifts. They compared the figures with the difference seen by age group at baseline and concluded that more than 10 years of rotating shift work was equivalent to 6.5 years of age-related decline. A similar difference was seen for the memory score, but not the speed processing score.
There were no significant differences in cognitive scores for people with 10 years or less exposure to rotating shift work compared to those who never had worked rotating shifts.
People who were currently working rotating shifts had an equivalent of 5.8 years of age-related decline, and people who had left within the past five years had an equivalent of 6.9 years of age-related decline compared to those who never had worked rotating shifts.
In contrast, those who had left rotating shifts more than five years before had no difference in cognitive tests compared to those who had never worked rotating shifts.
How did the researchers interpret the results?
The researchers concluded that, “exposure to shift work was associated with a chronic impairment of cognition; the association was highly significant for exposures to rotating shift work exceeding 10 years (with the exception of the speed scores among non-executive participants) and the recovery of cognitive functioning after having ceased any form of shift work took at least five years (with the exception of speed scores”. They also say, “the current findings highlight the importance of maintaining a medical surveillance of shift workers, especially of those who have remained in shift work for 10 years or more”.
The researchers conclude that, “shift work was associated with impaired cognition”, but as this was found at the start of the study, it cannot prove that shift work was the cause. It is possible that people who performed shift work differed in baseline cognitive ability from those who didn’t, which may be related to various other factors (such as educational attainment). To prove cause and effect, the study would need to assess cognitive ability in individuals before any exposure to shift work.
Further limitations of this study include that in each analysis, the control group considered never to have been exposed to shift work may actually have had up to 50 days of shift work per year. A more rigorous criteria for the control group, such as working no days of shift work per year, may have been more useful.
It's not possible to draw firm conclusions about the cause of the association seen, as there was such a wide range of shift work patterns grouped together. It is also not known the type of shift work undertaken (for example, whether in a professional or more manual occupation).
The conclusion that cognitive function recovers five years after stopping rotating shifts is also not proven by this study. The researchers performed this section of the analysis using the information obtained at baseline only. They did not compare the cognition of individuals during periods of rotating shift work with their cognition five years after stopping. They compared people who had stopped with people who were still doing rotating shifts. Therefore, this analysis does not take into account their natural cognitive abilities.
Finally, it is not known whether the small differences in cognitive functioning, memory and processing scores observed between shift workers and day workers, would have actually made any meaningful difference in terms of the person’s daily life and functioning.
Overall, this study demonstrates an association between shift work and poorer cognitive function scores, but it did not prove that shift work was the cause.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.