By Liza Baskin
Working overnight shifts can disrupt sleep patterns and potentially have negative effects on a person's long-term health. It's possible that mental function could be affected.
A recent study looked at the link between overnight shift work during midlife and cognitive (basic mental function) decline. The participants were female nurses who had worked different lengths of shift work throughout their lives.
The researchers found that these women did not experience more cognitive decline than the average level of decline associated with old age.
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The researchers concluded that these findings do not support an association between shift work and increased cognitive decline.
The lead author of this study was Elizabeth E. Devore, MD, from the Channing Division of Network Medicine at Brigham and Women's Hospital in Boston, MA.
The researchers selected 16,190 women from a previous study called the Nurses' Health Study. That study initially sent out a questionnaire in 1988 that asked the women about the total number of years that they worked overnight rotating shifts.
The women were then interviewed three times, every two years, between 1995 and 2000. At each interview, the women were given the same set of six different cognitive tests in order to track their rate of cognitive decline.
All of the women were between 58 and 68 years old at the time of the 1988 questionnaire. Those with a history of stroke were not included.
The researchers of the current study estimated the average rate of cognitive decline for women later in life by averaging the scores of all the participants on the cognitive tests at each of the four data collection times.
The researchers considered outside factors that may have affected cognition such as age, education, alcohol intake, smoking status, physical activity, living alone, presence of depressive symptoms, history of high blood pressure, sleep duration and use of tranquilizer medications.
The average age of the participants was 74.3 years old.
The researchers found that 38 percent of the participants did not do shift work, 48 percent worked shift work for one to nine years, 8 percent worked shift work for 10 to 19 years and 6 percent worked shift work for more than 20 years.
The characteristics of the participants was generally similar, except for the women who had worked shift work for over 20 years. These women reported lower levels of education than the other groups.
This group of women who had done shift work for more than 20 years was the only group found to have significantly lower scores than the other groups on the cognition tests.
However, the researchers discovered that there were no significant or consistent associations overall between shift work during midlife and cognitive decline in this population.
Based on these findings, the researchers suggested that a history of shift work did not affect older women later in life.
The authors mentioned a few limitations of their study.
First, the researchers did not ask women about their history of permanent night work, so some of the participants may have mistakenly reported their permanent night work as rotating, overnight shift work. People who always work the night shift tend to not have as many issues with sleep disruption -- an issue thought to be behind potential cognitive decline.
Second, the participants' histories were only reported for the midlife period and not after that. Shift work in later life could have affected their scores on the tests. Third, the participants who did not report a history of shift work had lower scores than the women who did. This may have skewed the results.
This study was published in the October edition of the American Journal of Epidemiology.
The National Institutes of Health provided funding.