A study of middle-aged and older men and women found that compared with non-shift workers, shift workers performed less well on a test that doctors use to screen for cognitive impairment. However, people who had not been shift-working for at least 5 years completed the test just as well as non-shift workers.
The study, by Uppsala University and Malmö University - both in Sweden - is published in the journal Neurobiology of Aging.
Corresponding author Christian Benedict, an associate professor in the neuroscience department at Uppsala University, says:
"Our results indicate that shift work is linked to poorer performance on a test that is frequently used to screen for cognitive impairment in humans."
The researchers analyzed data on around 7,000 men and women aged 45-75 years who are taking part in a large study looking at factors that affect the development of disease in the elderly in Sweden.
The study investigated the link between self-reported shift-work history and performance on a neuropsychological test called the "trail making test," which is commonly used to assess executive cognitive function - a core brain function that is known to decline with age.
Executive cognitive function is the set of mental skills that helps us get on with daily living and respond effectively as our environment changes. For example, we use it to plan, reason, and focus on a task.
Brain might take 5 years to recover from shift work
Tests of executive function are often included when assessing core brain functions - for example, when diagnosing dementia.
The trail making test has two parts. In the first part, you have connect randomly arranged circles labeled 1-25 in ascending order. In the second part, you have to alternate between numbers and letters in ascending order. Usually, the older you are, the longer it takes to complete the test.
The researchers found that people who were currently working shifts - or who had worked shifts in the previous 5 years - performed worse on the test than non-shift workers.
They found this after taking into account other factors that could influence the results - such as age, education, and sleep duration.
In contrast, the researchers found that people who had worked shifts up to 5 years before - but not since - performed just as well as non-shift workers.
"The latter could suggest that it may take at least 5 years for previous shift workers to recover brain functions that are relevant to the performance on this test."
Prof. Christian Benedict