Does night work increase the risk of osteoporosis?

An analysis of more than 270,000 UK adults shows that working all night, especially over the long term, is linked to weaker bones and a higher risk of fractures, highlighting an overlooked occupational health issue.

Study: Association between night shift work and the risk of osteoporosis and osteoporosis-related pathological fracture. Image credit: Pixel-Shot/Shutterstock.com

A recent study in Frontiers in Public Health examined the association between night work and osteoporosis (OP) and related fracture risks. Compared with daytime workers, those who work regular or permanent night shifts and, to a lesser extent, people with previous night shift exposure, are associated with an increased risk of developing osteoporosis.

Prevalence and factors leading to osteoporosis.

Osteoporosis (OP) is a widespread systemic skeletal disorder characterized by decreased bone density and structural deterioration that makes bones weak, brittle, and susceptible to fractures. Since this condition can develop without any symptoms, it is considered a “silent disease.” In most cases, OP occurs in the hip, spine, or wrist.

Several factors accelerate bone degradation, including age-related bone loss and hormonal changes, particularly the decline in estrogen during menopause. Additionally, genetic predisposition, low body mass index (BMI), long-term use of certain medications, and lifestyle choices, including smoking, excessive alcohol consumption, and physical inactivity, increase the risk.

According to the National Center for Health Statistics (NCHS), more than half of American adults age 50 and older have OP or are at increased risk due to decreased bone mass. The American Academy of Orthopedic Surgeons (AAOS) emphasizes that effective primary prevention strategies, such as physical activity, lifestyle modifications, and, in some cases, orthopedic medications, can significantly reduce the incidence of OP by mitigating bone mineral density loss. Given this burden, identifying new risk factors for osteoporosis remains a critical research priority.

The effect of shift work on health

Human physiological functions are regulated by an inherent circadian rhythm that optimizes biological processes in alignment with daily environmental and behavioral patterns. Shift work, defined as employment outside of conventional daytime hours, significantly disrupts circadian rhythms and sleep cycles. Night shift work is generally defined as working at least three hours between midnight and 6 a.m.

Variable shift work has increased significantly in developed countries. For example, previous research estimates that approximately 21% of EU and 29% of American employees work shifts.

Multiple studies have shown that long and frequent night work shifts can alter metabolic functions and hormone secretion, thereby increasing susceptibility to chronic diseases. While research links night shifts to obesity, aging problems and heart disease, the connection to OP remains less established.

Assess whether night work influences OP

The current study recruited employed or self-employed participants with exposure to the night shift from the UK Biobank, which contains over 500,000 people aged approximately 40 to 69 years. Any participant with pre-existing osteoporosis was excluded.

At baseline, participants reported their work schedules and were classified as either day workers, who worked from 9 a.m. to 5 p.m., or shift workers, who worked in the afternoon, evening, night, or in a mixed shift rotation. According to the frequency responses, they were further grouped into day workers, shifts but never or rarely night shifts, some night shifts, or regular or permanent night shifts.

Lifetime employment data were used to assess whether duration, total years, and frequency (average number of night shifts per month) of night shift work correlate with OP risk, the primary outcome. Additionally, this study evaluated whether night shift work interacts with genetic predisposition to influence susceptibility to OP. The researchers also evaluated the association between night shift work and OP-related pathological fractures, the secondary outcome. Cox proportional hazards models were used to evaluate hazard ratios (HRs) for the association between current night shift work and incident osteoporosis.

Current and past night shift increases OP risks

After screening, 276,774 participants with 5,906 OP events were analyzed: day workers (82.7%), shift workers who rarely worked nights (8.5%), some night shifts (4.9%), and regular or permanent night shifts (3.8%). Night shift workers were more likely to be male, younger, less educated, with longer working hours, greater socioeconomic disadvantage, non-European ethnicity, higher BMI, high prevalence of diabetes, shorter sleep hours, and later chronotypes.

Cox models showed that higher night shift exposure categories were generally associated with higher OP risk, with regular night shift workers having the highest risk. Multivariable models confirmed this trend, although the strongest and most statistically robust association was observed among workers on regular or permanent night shifts. Restricting analyzes to OP incidents that occurred two or more years after baseline strengthened the association. Regular night shift work also increased the risk of OP-related pathological fracture, with an HR of approximately 1.9.

The analysis of rotating night shifts before baseline included 75,120 participants, of whom 806 had OP. When lifetime exposure was assessed by total duration, cumulative years of night shift work was generally positively associated with the odds of OP. In the model adjusted for age, sex, and BMI, participants who worked night shifts for less than five years had a higher probability of OP than workers who never worked.

Multivariable adjustment suggested a higher risk with longer exposure durations, although estimates for more than 10 years of night shift work were modest and not statistically significant. When lifetime exposure was assessed by frequency, working an average of 3 to 8 night shifts per month showed the clearest increase in OP risk compared to those who never worked, highlighting a nonlinear pattern of risk.

Sensitivity analyzes adjusting for missing values, chronic disease, cancer, sleep factors, time outdoors, supplements, and female-specific variables did not materially modify these associations, supporting the strength of the observed relationship between night shift and OP. Stratified analyzes found no significant interactions between sex, BMI, sleep chronotype, sleep duration, or other AAOS confounders and current or lifetime night shift status on OP risk, indicating that the associations were broadly consistent across these subgroups.

Polygenic risk score (PRS) was positively correlated with OP risk, indicating higher risk with higher PRS. No significant interactions were found between genetic susceptibility and night shift variables for OP risk.

Conclusion

This study demonstrates a significant positive association between habitual or long-duration night work and an increased risk of osteoporosis and OP-related pathological fractures, independent of genetic susceptibility. As an observational analysis, the findings indicate association rather than causation, but still highlight the potential value of targeted bone health screenings and preventive interventions among night shift workers.

Although the relative risk was high, the absolute risk of osteoporosis for any individual worker remained low. These results suggest that occupational health policies could consider incorporating regular bone density monitoring and lifestyle modifications to help mitigate the risk of osteoporosis in this vulnerable population.

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