People who work night shifts for a decade show poorer memory and other cognitive impairments compared with day workers, say researchers, who also found recovery is possible.
Shift work, like chronic jet lag, disrupts the body's internal clock and has been linked to physical health problems such as cardiovascular disease and some cancers. Less is known about its potential effects on cognitive abilities.
To find out, researchers followed more than 3,200 employed and retired workers in three regions of southern France who had annual medical assessments from 1996 to 2006. Just under half of the workers had worked shifts for at least 50 days of the year.
The participants did tests of their memory and speed of processing on three occasions. They were 32, 42, 52 and 62 years of age when the first data was collected.
"Shift work chronically impairs cognition, with potentially important safety consequences not only for the individuals concerned, but also for society," Philip Tucker of the psychology department at Swansea University in Wales and his co-authors concluded in Monday's online issue of the journal Occupational & Environmental Medicine.
Most shift workers become progressively sleep deprived, partly because of the difficulty in sleeping well during daytime hours. (Akira Suemori/Associated Press)
The effects of shift work on cognition were equivalent to six years of normal cognitive decline from aging, Tucker said.
Researchers aren't sure why, but they suspect it's as if shift workers are constantly jet-lagged.
"It may be that the desynchrony, if you like, between people's work schedules and what their body clock would like them to do is actually affecting their brain structures," Tucker said in an interview.
It's increasingly recognized that sleep deprivation has significant effects on cognition, said Dr. Brian Murray, a neurologist and sleep specialist at Sunnybrook Health Sciences Centre in Toronto.
One of the reasons we sleep is to remove metabolic debris, said Murray. "A space in the brain opens up to facilitate clearance of these toxic products. If people don't sleep well, they're more likely to develop neurodegenerative diseases."
Sleep quality differences
Most shift workers become progressively sleep deprived because daytime sleep tends not to be as good as that obtained at night, said Murray, who was not involved in the study.
For example, the brain's hippocampus has actively dividing cells. One hypothesis is that during a stressful situation, cortisol is released that could impair function in the hippocampus, and that can impair memory formation, Murray said.
Tucker said the good news is that subjects also showed signs of recovery. "Those signs didn't occur until at least five years after giving up shift work."
The study's authors suggested that since the problems don't kick in until people worked in shifts for at least 10 years, perhaps workers could look for another job before that.
Mitigating night-shift work
Mohammed Sadique of Toronto drives a cab overnight and drinks coffee to stay awake. "I don't have to look at any map or GPS," he said of his memory.
But Sadique said he doesn't have much choice about when he works.
For employers and employees, experts suggest some ways to mitigate the effects, including :
- Set work schedules that involve only a few nights at a time so that the body clock remains in a relatively day-oriented state.
- Rotate shift work schedules in a clockwise fashion, which is easier to adapt to than counterclockwise.
- Get adequate sleep time when possible in a dark, quiet room.
- Encourage family and friends to respect sleep time.
The researchers acknowledged limitations of the study, such as the need to group some types of atypical work schedules and the possibility that there was some shift work in the control group.
The study's authors also said it's possible that those who quit shift work a long time ago may have higher cognitive abilities and were able to move into non-shift jobs at an earlier stage of their career.
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