Shift work is common in many occupations in modern society, particularly in service industries including healthcare, manufacturing and transportation. According to the Current Population Survey,¹ approximately 18% of the US labour force works alternative shifts that fall outside of a traditional day shift. While studies indicate that shift work (a proxy for light exposure at night and chronodisruption) may have adverse effects on acute health-related outcomes (eg, accidents, reproductive factors, gastrointestinal malfunction and sleep),^2–5 associations between night shift work and chronic diseases are less clear and difficult to study given the need for detailed exposure assessment and long follow-up to ascertain chronic disease out-comes.⁶⁷ A recent study⁸ of over 40 000 UK women found that working at night was associated with higher odds of smoking, obesity and being in the lowest third of socioeconomic status (SES), suggesting a more adverse risk profile for chronic diseases among shift workers.

Adding to this data, we examined the distribution of risk factors for cancer and cardiovascular disease—the most common chronic diseases and leading causes of mortality in the USA⁹—by night shift work history in the Nurses' Health Study (NHS) II, a large cohort of women with substantial shift work exposure, long-term follow-up, and a large number of reported health and lifestyle characteristics. Given the consistency of night shift work from young adulthood through older age among NHS II participants, we are in a unique position to evaluate night shift work and timing by specific age ranges in relation to chronic disease risk factors.

NHS II was initiated in 1989 when 116 430 female registered nurses, aged 25–42 years and living in 14 US states, completed an initial questionnaire on their medical history, health and lifestyle. Since 1989, similar questionnaires have been completed biennially to update this information, with follow-up rates at approximately 90%. This study was approved by the Institutional Review Board (IRB) of Brigham and Women's Hospital (Boston, Massachusetts, USA).

Table 1 describes occupational characteristics across age ranges that shift work was performed among 54 724 women in NHS II. Qualitatively, there were several important differences across these age ranges; specifically, as age at shift work increased, women were more likely to work in nursing careers outside of the hospital (3.3% among women aged 20–25 vs 16.5% among women aged 46+), more likely to have a primary day/evening work schedule (44.6% among women aged 20–25 vs 80% among women aged 46+), and less likely to work full-time (84% among women aged 20–25 vs 72.7% among women aged 46+). Additionally, more women worked rotating night shifts earlier in life compared to later in life (36.6% among women aged 20–25 vs 5.3%, among women aged 46+).

After adjusting estimates for age and SES and applying a Bonferonni correction, we observed several significant differences across chronic disease risk factors comparing ever vs never night shift workers (table 2). Among modifiable risk factors, ever night shift workers were more likely to have a higher BMI (28.1 vs 27 kg/m²), higher daily caloric (1822 vs 1772 kcal/day) and caffeine intake (184 vs 167 mg/day), shorter sleep duration (67% vs 63% ≤7 h/day), and to currently smoke (7% vs 5%) compared to never night shift workers. Among non-modifiable risk factors, compared to never night shift workers, ever night shift workers were more likely to be nulliparous (19% vs 17%), live alone (13% vs 11%) and less likely to be morning chronotypes (33% vs 38%). Among health-related factors, ever night shift workers were also more likely to have a history of diabetes (8% vs 6%), angina (3% vs 2%) and high blood pressure (36% vs 33%); regularly use antihypertensive medication (34% vs 32%) and aspirin (11% vs 9%), and less likely to have had a mammography in the past 2 years (90% vs 92%), compared to never night shift workers (table 2).

In table 3, we observed that night shift workers had significantly higher odds of obesity (defined as BMI ≥30 kg/m²; OR=1.37, 95% CI 1.31 to 1.43), higher caffeine (defined as ≥131 mg) (OR=1.16, 95% CI 1.12 to 1.22) and total calorie intake (defined as ≥1715 kcal per day; OR=1.09, 95% CI 1.04 to 1.13), less sleep (defined as ≤7 h per day; OR=1.19, 95% CI 1.15 to 1.24), and being a current smoker (OR=1.30, 95% CI 1.19 to 1.42) compared to never night shift workers. Further, odds of modifiable risk factors were generally stronger among ever nights only shift workers at any age range compared to ever rotating night workers, and varied depending on age at which night work was performed (tables 4 and 5). Our data suggests that rotating night work was significantly associated with BMI earlier in life (age 26–35) and later in life (age 46+) and smoking behaviour earlier in life (age 26–35) and mid-life (age 36–45; table 4). Nights only shift work was associated with BMI and smoking across almost all age ranges of working night only shifts; however, total calories, caffeine intake and sleep were generally not associated with night only work at any age ranges (table 5). In general, odds of risk factors were lower among women who worked rotating or night only shifts early in life (age 20–25), although this pattern was not entirely consistent.

In models that additionally adjusted for work stress, social support, type of nursing occupation and total years of night shift work, results did not substantially change (data not shown). Higher levels of average night shifts per month were significantly associated with an increased risk of obesity (eg, multivariable OR (95% CI) for 1–2 nights/month vs 0 nights/ month=1.13 (0.70 to 1.83); 3–4 nights/month vs 0 nights/ month=1.38 (0.85 to 2.22), 5–6 nights/month vs 0 nights/ month=1.65 (1.02 to 2.66), 7–8 nights/month vs 0 nights/ month=1.85 (1.15 to 2.99), 9–10 nights/month vs 0 nights/ month=1.85 (1.14 to 3.00), 11–15 nights/month vs 0 nights/ month=2.25 (1.39 to 3.64), 16–20 nights/month vs 0 nights/ month=2.28 (1.39 to 3.75), 21+ nights/month vs 0 nights/ month=3.42 (1.95 to 6.03); p-trend<0.0001). Furthermore, the increased risk of obesity was present across age categories among rotating and night only shift workers.

In this study of US female nurses, women were more likely to report a primary rotating night-shift schedule, full-time work and a nursing occupation within a hospital setting earlier in life compared to later in life. We also found that night shift workers tended to have a more adverse risk profile for chronic diseases compared to never night shift workers, and risk factors varied depending on age at which night work was performed.

Epidemiological studies, including previous studies in the NHS cohorts,^15–17 have observed differences in several established cancer and cardiovascular risk factors among those with versus without a history of shift work, including a higher BMI,^18–24 dietary intake²⁵ and smoking,^2126–29 as well as shorter duration of sleep.³⁰³¹ Exact mechanisms between shift work and these key risk factors are not well established. Several putative mechanisms have been suggested, including increased circadian rhythm disruption and disturbed sociotemporal patterns^5183233; both can lead to behavioural changes in dietary intake, smoking and duration of sleep. These key risk factors may lead to unfavourable metabolic disturbances contributing to increased obesity among night shift workers. In line with our results, a recent study⁸ that examined characteristics of 41 652 women in the Million Women Study, a cohort with 13.2% of participants reporting night shift work, found that women with a history of night shift work had a higher BMI and were more likely to be current smokers compared to those without a history of night shift work (mean BMI=27.3 vs 26.6 kg/m²; percent smokers=8.6% vs 6.2%, respectively). This study did not assess the association between night shift work and daily caffeine or total calorie intake.

Interestingly, we found that higher intensity of night shift work was associated with an increased risk of obesity, suggesting that increasing average number of night shifts worked per month (possibly indicative of more severe circadian disruption) might be an important risk factor for obesity among shift workers. This is also reflected in the stronger ORs for obesity we observed among women who reported night only shift work as their primary work schedule (table 5), compared to those with rotating night shift work as their primary work schedule (table 4). Additional studies are warranted to confirm our finding, and to also explore the role of individual chronotype in these associations; that is, whether a shift schedule that is in line with a person's chronotype (eg, night work carried out by evening types) may less severely affect obesity and related outcomes than a shift schedule opposing chronotype (eg, early morning work schedules for morning types).

While it appears plausible that the age at which an individual performed shift work may modulate the effects of shift work on health outcomes, no other prior study, to our knowledge, has examined this particular aspect of shift work among women. Previous data from the NHS cohorts indicate that longer durations of shift work are associated with higher cancer and CVD risk, whereas shorter durations do not appear to increase these risks.^15–173435 This data could provide indirect evidence for a higher risk associated with shift schedules worked earlier in life since duration could simply be an indicator of exposure earlier in life. However, in our current study, although adverse risk profiles appeared to vary slightly depending on the age at which a woman worked night shifts, and were stronger among women with night work only, versus those with rotating night shift worker, no clear pattern emerged in support of this.

The observed differences we found in chronotype among shift workers (ie, a lower proportion of morning chronotypes among night shift workers) were consistent with previous publications.⁸³⁶³⁷ Recent studies³⁶³⁷ have provided evidence that evening or ‘neither’ (ie, neither morning nor evening) chronotype may be associated with an increased risk of breast cancer compared to morning chronotype. While further research is warranted to validate these findings, it is nevertheless interesting that we observed a lower prevalence of morning types and less favourable health characteristics among night shift workers.

Strengths of this study include its size, wide variety of health and lifestyle characteristics, and detailed shift work information specific to the ages when work was performed. There are also several limitations of this study. First, these analyses are cross-sectional and do not necessarily have a causal interpretation. However, our results are intended to inform future prospective studies in exploring associations between night shift work and chronic disease risk. Second, our analyses utilise information on women's risk factor profiles from later in life, and therefore shorter term versus Longer term effects of shift work are not differentiated. Third, assessment of shift work was self-reported and ascertained retrospectively; hence, this exposure is susceptible to random misclassification. To alleviate some of this concern, we excluded women with inconsistent shift work histories in our analysis. If any misclassification is present, it is likely non-differential; however, given the nature of shift work, others have also reported the possibility of differential misclassification.³⁸³⁹ Finally, the study population consists primarily of white female nurses and may not be generalisable to other populations, such as men and different racial groups. Future studies should explore associations between shift work and chronic disease risk factors in diverse populations.

Taken together, our findings suggest that night shift work may be correlated with an adverse risk profile for chronic disease and that differences in risk profiles may exist, depending on the age range at which night shift work occurs. Differences in obesity risk suggested the most clinical relevance. Future prospective research is needed to confirm if timing of night shift work may impact adverse health effects. If confirmed, these findings may indicate that targeted lifestyle interventions for night shift workers should take into account the age at which night shift work starts and/or occurs, as risk factors may vary over the course of a woman's shift work career.