Occasional, or rotating, night shift work, even if done over a decade, does not seem to be directly linked to a higher risk of multiple sclerosis. But working nights for more than 20 years — and likely beginning such shifts early in a career — carries an almost three times higher risk of a definitive MS diagnosis, results of a large-scale study in nurses suggest.

These findings were reported in “Rotating night shift work and risk of multiple sclerosis in the Nurses’ Health Studies,” published in the journal Occupational and Environmental Medicine.

Changes in sleep and waking patterns are known to have an important impact on the body. The amount of time a person is or is not exposed to sunlight can modulate the way cells and tissues behave through a natural control mechanism called the circadian clock.

Maintaining a healthy 24-hour circadian rhythm is increasingly recognized as an important contributor to overall good health. Growing evidence suggests that people with a disrupted internal or biological clock, and are exposed to light during nighttime hours — such as night-shift workers — are at greater risk of chronic diseases such as cancer, cardiovascular disease, and MS.

The impact of night shift work could explain some events that trigger and support MS progression. However, the association between night shift work and MS risk remains under-explored.

Austrian and U.S. researchers reviewed the clinical records of a total of 198,419 women who participated in the Nurses’ Health Study (NHS) and the subsequent NHSII. (A third such study is now beginning; information on how to participate can be found here.)

NHS opened in 1976 and the NHSII study in 1976 and 1989; both recruited female nurses in the U.S. All participants were asked to complete a questionnaire every two years, giving updated information on lifestyle and their overall health or medical conditions.

Follow-up data revealed 579 MS cases confirmed by a physician — 109 cases among women taking part in the NHS study, and 470 among those in NHSII. Of these, 407 cases had a definite MS diagnosis and were used for this study.

A detailed analysis did not show a significant association between the number of years of rotating night shifts (defined three or more nights per month) and MS risk, with night shifts reported at the time of study enrollment or in updated history. These findings were consistent within both NHS and NHSII.

But data from NHSII showed that nurses who worked rotating night shifts for more than 20 years were 2.6 times more likely to have a definite MS diagnosis. This outcome was not influenced by weight or smoking status at the study’s start.

“Overall, we found no association between rotating night shift work history and MS risk in these two large cohorts of nurses,” the researchers wrote. Still, “MS risk tended to increase with a longer history of shift work (20+ years) suggesting that long-term or early career circadian disruption might be critical for MS.”

They noted, however, that their “estimates were based on small case numbers, and therefore have to be viewed with caution.”

Based on their findings, researchers suggest that “night workers may benefit from preventive measures and disease screening programs at the workplace, already at early career stages.”