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Working nights or odd hours can sap mental function, study finds

Working nights or odd hours can sap mental function, study finds
A new study finds that people who work nights or have irregular schedules that are at odds with their body's natural circadian rhythm showed signs of reduced cognitive function. (Glenn Koenig / Los Angeles Times)

Working an irregular schedule that includes afternoon and night shifts can seriously sap your brain power, new research shows.

For people who spent more than a decade on this type of rotating shift schedule, the effects were equivalent to 6.5 years of normal age-related cognitive decline, according to a study in the journal Occupational & Environmental Medicine.

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Even people who spent at least 50 days in a single year working any type of irregular work schedule – including regular shifts that prevented them from going to sleep before midnight or that forced them to wake up before 5 a.m. – suffered a significant mental toll. Tests revealed that these workers experienced the equivalent of 4.3 years of age-related cognitive decline, on average, even if their shift work happened years ago.

However, the researchers did find evidence that people's brains could recover after their work schedules went back to normal. People who hadn't worked rotating shifts for at least five years had cognitive test scores that were about the same as people who had never worked an irregular schedule.

But people who were currently working a rotating schedule had worse scores – the equivalent of 5.8 years of age-related decline. Even worse were people who had stopped working a rotating schedule within the last five years; their scores indicated the equivalent of 6.9 years of age-related cognitive decline.

When the researchers expanded the analysis to include people who experienced any kind of irregular work schedule, the results were similar.

All of the data came from VISAT, a long-term study of salaried workers from southern France.  The workers, who held a wide range of jobs, were given questionnaires and clinical exams up to three times over a 10-year period. A total of 3,232 workers were included in the study; of them, 1,484 (46%) said they had worked irregular shifts at some point in their careers.

Previous studies have found that work schedules that are out of sync with the body's natural circadian rhythm can harm mental function. For instance, airline crews that experienced frequent jet lag and didn't get enough time to recover got poor results on tests of their cognitive abilities. Industrial workers assigned to odd shifts did worse on memory tests compared with their counterparts who only worked days. And nurses who sometimes worked nights got lower marks on tests of general cognition.

The authors of the new study wrote that their results may be additional evidence that messing with circadian rhythms can cause "physiological stress, which has been shown to have an impact on brain structures involved in cognition and mental health."

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