Night shift work and mental health among female nurses: the moderating role of age in a large cross-sectional study

For decades, occupational health researchers have understood that working while the rest of the world sleeps takes a toll on the human body. The human circadian rhythm, our internal biological clock that regulates sleep and wakefulness, is designed to align with the sun. When nurses work night shifts, they force their bodies to function against this natural cycle, a disruption consistently linked to higher rates of depression, anxiety, and poor sleep. While the general harm of night work is well-established, a specific question has remained unanswered: does this harm change as a woman ages? Specifically, does the hormonal transition of menopause make night shifts more dangerous for mental health, or is the effect driven simply by the accumulation of years on the job? This distinction matters because it determines whether hospitals should focus on age-based scheduling or hormone-based accommodations to protect their staff.

A team of researchers at the First Hospital of Jilin University in China set out to answer this question by looking at the lives of 4,002 female nurses. They conducted a large survey between March and June 2024, asking these women about their work schedules, their sleep, and their mental well-being. The researchers measured depression and anxiety using standard questionnaires that ask about feelings of sadness, worry, and loss of interest, and they assessed sleep quality by asking about how long it takes to fall asleep, how often one wakes up, and how rested one feels in the morning. They also asked every participant to classify their menopausal status, dividing them into those who were still menstruating and those who had reached menopause or were in the transition phase. The goal was to see if the negative impact of night shifts was stronger for older, post-menopausal women compared to their younger colleagues.

The results confirmed what many suspected: working nights is associated with worse mental health outcomes across the board. Nurses who worked at least one night shift a month reported significantly higher levels of depression, anxiety, and sleep disturbances than those who worked only during the day. The data showed a clear dose-response relationship, meaning that the more nights a nurse worked, the worse their symptoms tended to be. For example, among the younger nurses, those working nights were nearly twice as likely to experience clinically significant sleep problems compared to day workers. This suggests that the disruption of the circadian rhythm is a powerful stressor that affects the mind regardless of other factors.

However, the study found something surprising when it looked at the role of menopause. The researchers had hypothesized that the hormonal changes of menopause might make older women more vulnerable to the stress of night shifts. The data did not support this. While the older nurses did report different baseline levels of some symptoms, the difference between night workers and day workers did not get larger as women got older or entered menopause. In fact, the statistical test for an interaction between menopause and night shifts was not significant. This means that the hormonal state of menopause itself does not appear to be the key factor that amplifies the harm of night work.

Instead, the study pointed to age as the more important factor, but in a way that suggests resilience rather than vulnerability. When the researchers analyzed the data by age, using a cutoff of 45 years, they found a significant difference in how night shifts affected depression. The negative impact of night work was strong and clear for nurses under 45. For nurses aged 45 and older, the difference between night workers and day workers disappeared; the night workers in this older group did not show significantly worse depression scores than their day-working peers.

The authors suggest this pattern is likely due to a phenomenon known as the "healthy worker effect." This concept describes how, over time, the most vulnerable people in a workforce tend to leave the most demanding roles. In this context, it implies that the older nurses who are still working night shifts are a highly selected group. They may be the ones who have naturally adapted to the schedule, have developed better coping strategies over years of experience, or have simply survived the selection process because they were never as affected by the disruption as their peers who moved to day shifts or left the profession entirely. The women who found the night shifts too difficult for their mental health likely stopped working nights long before they reached menopause. Therefore, the lack of harm seen in the older group is not because menopause protects them, but because the most sensitive individuals are no longer in that specific group.

The study also highlighted that the impact of night shifts is most acute for younger nurses. Among pre-menopausal nurses, working nights increased the risk of clinically significant depression by nearly 70 percent compared to day workers. The researchers calculated that for every eight younger nurses assigned to night shifts, one additional case of significant depression could be expected compared to a group working only days. This finding shifts the focus of occupational health interventions. Rather than assuming that older, menopausal women are the most at risk, the data suggests that younger nurses are the most vulnerable to the mental health consequences of night work.

Ultimately, this research clarifies that while night shifts are undeniably harmful to the mental health of female nurses, the mechanism is not driven by the hormonal changes of menopause. The apparent protection seen in older nurses is likely a result of who remains in the night-shift workforce, not a biological shield provided by age or menopause. The study concludes that hospital administrators and health policymakers should prioritize reducing the frequency of night shifts for all nurses, with a specific emphasis on protecting younger workers who have not yet had the time to adapt or who have not yet been filtered out by the demands of the job. By focusing on the schedule itself rather than the age or hormonal status of the worker, healthcare systems can better address the root cause of the distress.