Shift work has become more prevalent across a wide range of occupations, such as health care, manufacturing, and transportation, in part due to increased demand for 24-h services and technological advances. Generally, shift work is defined as working outside the normal daytime working hours (07:00h and 18:00h) and can include evening shifts (14:00h and 00:00h), night shift (19:00h and 07:00h) or rotating between day, evening, and night shifts (Caruso 2014; Depner et al. 2014). In the United States, nearly 20% of employed adults are engaged in shift work (Caruso 2014; McMenamin 2007). According to the Bureau of Labor Statistics (BLS), 16% of U.S. workers usually worked on a non-daytime schedule that includes evenings, nights, split or extended shift, or rotating shifts (BLS 2021).

Findings from a growing number of studies indicate shift work may lead to numerous adverse health outcomes. For example, a 2016 umbrella review of 38 meta-analyses and 24 systematic reviews indicated significantly increased risk overall for coronary heart disease, stroke, Type 2 Diabetes, specific types of cancer, injury, and reduced cognitive function (Kecklund and Axelsson 2016). In a more recent meta-analysis regarding the impact of shift work on cardiovascular disease, Torquati and colleagues found the risk of coronary heart disease (CHD) morbidity to be 26% higher in shift workers than those on day shift (RR = 1.26, 95% CI: 1.10–1.43) and that, after the first 5 y of shift work, the risk for CVD events increased by 7.1% for every additional 5 y of exposure, suggesting a dose–response relationship (Torquati et al. 2018). The 2019 designation of night shift work as “probably carcinogenic to humans” by the International Agency for Research on Cancer underscores the growing recognition of shift work as a significant occupational hazard (Koritala et al. 2021; Sweileh 2022; Ward et al. 2019). However, the potential relation of shift work to leisure-time physical activity (LTPA), which may have the potential to mitigate the adverse effects of shift work (Flahr et al. 2018), remains little explored in the U.S. workforce, especially on a national level (Loprinzi 2015).

Leisure-time physical activity (LTPA) is a major modifiable health behavior, with compelling evidence demonstrating the health-promoting effects contributing to chronic disease prevention (Lee et al. 2012; Warburton and Bredin 2019; Warburton et al. 2006), including for cardiovascular disease (Lee et al. 2003; Li et al. 2006, 2013; Pedersen et al. 2008; Rodriguez et al. 1994), type 2 diabetes (Folsom et al. 2000), cancer (Friedenreich and Cust 2008; Huxley et al. 2009), and osteoporosis (Berard et al. 1997). A systematic review of prospective cohort studies showed a significant positive relationship between sedentary time and all-cause mortality (Ekelund et al. 2019); subjects in the highest quartile of sedentary time (most sedentary) were nearly 3 times more likely to die of all-causes relative to those who were least sedentary (Hazard ratio = 2.63, 95% CI: 1.94–3.56). As documented in a recent World Health Organization report, people who are insufficiently active have a 20% to 30% increased risk of premature death compared to those who are sufficiently active, and physical inactivity is a leading risk factor for chronic disease mortality (World Health Organization 2020). Furthermore, evidence suggests that LTPA is associated with a decreased risk of cognitive decline (Sabia et al. 2017; Sofi et al. 2010) and may provide psychological and social benefits (Eime et al. 2013). To achieve the health benefits of PA, current global guidelines recommend a minimum of 150 min of moderate intensity physical activity each week (moderate for 150+ minutes/week or vigorous for 75+ minutes/week) (Bull et al. 2020; U.S. Department of Health and Human Services 2018). Despite these well-documented health benefits, many adults fail to achieve the recommended physical activity guidelines, especially those engaged in shift work (Monnaatsie et al. 2021). However, studies regarding the impact of shift work on LTPI are limited. According to recent data for the year 2022, inactivity level was 26.3% among U.S. adults (Healthy People 2030), while less than half (48.1%) engaged in sufficient regular physical activity.

Despite robust but disparate lines of research on the relationship between shift work and LTPA and health, there is a need for a comprehensive assessment regarding the prevalence of leisure-time physical inactivity (LTPI) within the U.S. workforce, and the potential link between shift work and LTPI. The literature on the relationship between shift work and leisure-time physical activity remains limited. To our knowledge, only one published epidemiologic study has examined the impact of shift work on physical activity in a national sample of the U.S. labor force (Loprinzi 2015), although this investigation did not distinguish between leisure-time and occupational physical activity. Moreover, as highlighted in a recent systematic review and meta-analysis (Monnaatsie et al. 2021), results from prior studies comparing PA and sedentary behavior (SB) in shift workers versus daytime workers have been inconsistent; while some studies have reported a significant negative impact of shift work on PA (Loprinzi 2015, Nabe-Nielsen et al. 2011), others have indicated non-significant differences in PA (Hulsegge et al. 2017; Loef et al. 2018; Monnaatsie et al. 2021), while still others have reported shift workers to be more active than daytime workers (Loprinzi 2015; Marqueze et al. 2013; Monnaatsie et al. 2021). In addition, the limitations characterizing most previous studies render interpretation and/or generalization of findings challenging. The limitations include lack of sex-stratified results, small sample sizes, lack of occupational diversity, failure to distinguish between leisure-time and occupational PA, and other factors. To improve our understanding of the potentially complex relationships between shift work and LTPA, the central goal of this study was to examine the associations of shift work status (yes/no) and shift schedule (day, evening, night, rotating, and some other schedule) to leisure-time physical inactivity (LTPI) in a nationally representative sample of U.S. workers. Because gendered roles such as caregiving or other household responsibilities could affect LTPI levels (Woodward 2019), analyses were stratified by sex.

This study sought to examine the association of shift schedule and shift work status to leisure-time physical inactivity (LTPI) in a nationally representative sample of employed men and women. Our findings indicate that 47% of the U.S. labor force did not meet recommended levels of leisure-time aerobic physical activity, including 26% who were physically inactive. In the overall workforce, LTPI was 13% higher among night shift workers compared to those on regular daytime shifts. Furthermore, sex modified the association between shift schedule and LTPI; among females, working the evening shift was associated with a 24% increased prevalence of LTPI and working the night shift was associated with a 17% increased prevalence of LTPI, after accounting for demographic characteristics, health behaviors, and work hours. Similarly, female shift workers (those on evening, night, rotating, and other schedules combined) had a 13% higher prevalence of LTPI compared to their counterparts working regular daytime shifts. In contrast, the association of shift work status to LTPI was not significant in male workers. The results suggest that shift work may be an important risk factor for LTPI, especially among female workers.

While the exact biological mechanisms are not entirely understood, it is commonly hypothesized that associations between shift work and negative health outcomes (e.g. CVD and diabetes) result from circadian rhythm disruptions and associated sleep disturbances (Arendt 2010; James et al. 2017). Persistent circadian misalignment and sleep restriction could impair several physiological processes that contribute to the development of cardiometabolic diseases (Kervezee et al. 2020). In addition, behavioral risk factors (e.g. physical inactivity) have been hypothesized to mediate the relationship between shift work and adverse health outcomes (Hulsegge et al. 2017). However, findings from previous studies comparing physical activity between shift workers and daytime workers have been inconsistent (Monnaatsie et al. 2021).

Some studies reported no significant differences in leisure-time physical activity levels between shift workers and daytime workers. A large epidemiologic study (Loef et al. 2017; n = 6,512), based on self-reported assessment of PA, reported that shift workers spent more time walking than non-shift workers, but there were no significant differences between the two groups with respect to other types of leisure-time PA (e.g. cycling, chores, other moderate-to-vigorous exercises. A study based on objective assessment of PA (Hulsegge et al. 2017; n = 812) reported that shift workers had similar leisure-time PA patterns as daytime workers (difference in percentage of time spent in moderate-to-vigorous leisure-time PA between night versus day shift workers = 0.1, 95% CI: −0.6–0.8). A study of health-care workers employed in hospitals (Loef et al. 2018; n = 479) concluded that objectively assessed leisure-time PA levels (walking, running, stair-climbing, and cycling) of shift workers were similar to those of non-shift workers. These results are consistent with our findings in the overall workforce and male workers but not in female workers. A recent meta-analysis comparing overall PA in shift workers versus non-shift workers (Monnaatsie et al. 2021) concluded that there was no statistically significant difference between shift workers and non-shift workers in terms of meeting physical activity guidelines (OR = 0.84, 95% CI: 0.68–1.03; pooled data from 12 studies) or time spent in moderate-to-vigorous physical activity (standardized mean difference = −0.1, 95% CI: −0.4–0.20; pooled data from 12 studies). It is worth noting, however, that some of the evidence by Monnaatsie and colleagues includes studies that did not distinguish between leisure-time PA and occupational PA and therefore it is not directly comparable to our study.

Results from other studies (Nabe-Nielsen et al. 2011; Ma et al. 2011; Kolbe-Alexander et al. 2019; Loprinzi 2015) appear to support the hypothesis that shift work is associated with decreased levels of leisure-time physical activity. For example, a study of U.S. workers based on 2005–2006 NHANES survey (Loprinzi 2015; n = 1,536) showed that shift workers (those on evening or night shift) were engaged in significantly less sustained (bouts of) moderate-to-vigorous PA (both occupational and leisure-time combined) compared to those working on regular daytime schedules. In a prospective study of Danish female health-care workers, fixed night work was associated with lower odds of becoming physically active at leisure-time (Nabe-Nielsen et al. 2011); among those who were sedentary at baseline, the odds of becoming physically active at the end of the 2-y followup was 92% smaller in those working on fixed night shift compared to those on a fixed day shift (OR = 0.08, 95% CI: 0.01–0.75). These results are congruent with our finding that, relative to female workers on daytime shifts, those on fixed night shift were significantly more likely to be physically inactive during their leisure time.

On the other hand, there are, albeit limited, data to suggest that, in certain populations, shift workers may be more active than non-shift workers (Loprinzi 2015; Marqueze et al. 2013). For example, in a national sample of American adults (Loprinzi 2015), workers on rotating shifts were found to engage in significantly more light intensity physical activity (including leisure-time and occupational PA combined) than daytime workers. Similarly, a small cross-sectional study of truck drivers (31 irregular shift workers, 26 daytime workers) reported that self-reported level of sufficient leisure-time physical activity (≥150 min/wk.) was higher among shift workers compared to daytime workers (25.8% versus 3.8%), while insufficient leisure-time physical activity (<10 min/wk.) was lower in shift workers compared to daytime workers (48.4% versus 88.5%) (Marqueze et al. 2013). Consistent with these findings, our data also indicate that male workers on rotating shift were significantly less likely to be physically inactive compared to those on daytime schedule (PR = 0.87, 95% CI: 0.78–0.96). More specifically, our data also indicate that among young (≤44 years) male workers, prevalence of LTPI was 23% higher in those working the regular night shift (PR = 1.23, 95% CI: 1.01–1.51) but 15% smaller in those working the rotating shift schedule (PR = 0.85, 95% CI: 0.74–0.97) compared to their counterparts working the regular daytime shift. It is possible that a rotating shift that enables workers to rotate between day, evening, and night shifts while allowing employees to take extended periods off work after prolonged assignment on unfavorable shifts could improve work-life imbalance and provide an opportunity to recharge and engage in leisure time physical activity.

Studies comparing specific LTPI measures of sedentary behavior (SB) between shift workers versus daytime workers are few, and findings have been inconsistent. In a study of working U.S. adults, Loprinzi (2015) reported that those on rotating shifts engaged in significantly less sedentary behavior (leisure-time and occupational combined) compared to daytime workers, a finding broadly consistent with our results in male rotating shift workers demonstrating a reduced likelihood of LTPI. However, while we analyzed leisure-time PA as a separate variable, Loprinizi combined leisure time with occupational PA, rendering comparisons between the studies challenging. Conversely, a Danish study (Hulsegge et al. 2017) reported no significant difference between daytime and shift workers in the percentage of time spent in sedentary behavior at leisure-time (61.1% for dayshift workers and 62.4% for night shift workers. A study of health-care workers (Loef et al. 2018) found no significant differences in proportion of leisure-time spent in sedentary behavior between shift workers and daytime workers (61.4% versus 59.5%). A recent meta-analysis (Monnaatsie et al. 2021) reported that time spent in sedentary behavior was significantly lower in shift workers than non-shift workers (pooled data from 7 studies with all but one assessing PA via a device). However, in this same study, meta-analysis using the six device-based studies showed no significant differences in sedentary behavior between shift and non-shift workers. Similarly, results from our study for the overall work force shows no significant difference in LTPI between non-daytime and daytime workers (PR = 1.02, 95% CI: 0.97–1.08). However, workers on night shifts were 13% more likely to be inactive compared to those on daytime schedule.

Overall, findings of our study suggest that sex may modify the effect of shift schedule and shift work status on leisure-time physical inactivity (LTPI). Females engaged in shift work are 13% more likely to be inactive compared to their counterparts on daytime shift while no significant differences were observed between the two groups in male workers. Specifically, females on evening and night shift are 24% and 17% more likely to be inactive compared to those on daytime shift, respectively, while no differences were found in males. Our study also indicates that males on rotating shift had 13% lower prevalence of LTPI compared to those on daytime shift. But among females, our data show that the prevalence of LTPI was 12% higher for those on rotating shift compared to those on day shift.

It is worth noting that there are several factors that could contribute to the inconsistencies in result between the current study and prior studies of shift schedule/shift work and physical activity including variations in (a) shift schedules compared (shift worker versus daytime worker, rotating versus daytime, evening versus daytime, night versus daytime, etc.), (b) type of occupation of the study participants (uniform versus diverse), (c) sample size, (d) type of PA (leisure-time versus occupational versus both combined) (e) definition of outcome measures of PA (prevalence of meeting the PA guideline versus time spent in moderate-to-vigorous activity versus time spent in sedentary behavior versus prevalence of being inactive), (f) measurement tools used for assessment of physical activity (device-based versus self-report), (g) questionnaires used for assessment of self-reported PA, and (h) cut points used for classifying physical activity (e.g. sustained versus overall moderate-to-vigorous PA). For example, Loprinzi (2015) reports significantly lower sustained moderate-to-vigorous PA in shift workers than those on regular daytime schedule but no differences between the two groups in terms of overall moderate-to-vigorous PA. Similarly, a study by Monnaatsie et al. (2021) reported significantly lower SB in shift workers compared to those on day shift (pooled data from 7 studies) but the difference between the two groups was no longer statistically significant when the analysis was restricted to the 6 studies that utilized devices for assessment of PA. Future studies comparing physical activity and sedentary behaviors in shift and daytime workers should consider standardized protocols for assessment of shift work and a device-based assessment of PA.

The major strength of this study is that it is based on a large, nationally representative sample that includes U.S. workers from a diverse array of industry and occupation categories. In addition, many covariates (demographic variables and health behaviors) were collected as part of NHIS, allowing us to adjust for multiple potential confounders. Our study distinguishes leisure-time PA from occupational PA and includes sensitivity analyses allowing assessment of potential modifying effects of age and sex. We acknowledge the following limitations. Work schedule, leisure-time physical activity, and covariate data are all self-reported. Subjective assessment of leisure-time physical activity via questionnaires may result in misclassification due to social desirability bias (Troiano et al. 2008) in which participants tend to overestimate their physical activity (Sylvia et al. 2014), difficulty in accurately assigning intensity to reported activities, and limited ability to recall frequency and duration of PA. Thus, confirmation of findings with objective device-based PA assessments is needed. The analysis was based on a cross-sectional assessment of participants’ work shift and leisure-time PA, limiting casual inference. Because shift work tenure was unspecified, we were unable to examine whether LTPI was higher among those with longer tenures.

In summary, this study presents evidence that shift work may contribute to leisure-time physical inactivity. Further, the relationship of shift work to leisure-time inactivity varied with sex. This study also adds to the body of knowledge regarding the continuing high prevalence of inactivity/insufficient activity in the U.S. workforce. Overall, our work has established firm associations between shift work and leisure-time physical inactivity, thereby highlighting the need for inclusion of physical activity in health promotion interventions to address the higher inactivity level among shift workers, especially among female workers. In conclusion, the findings from the current study supports the assertion by Loprinzi (2015) that physical activity and sedentary behavior interventions ought to be tailored based on shift schedules. Increasing leisure-time physical activity may have the potential to mitigate the adverse effects of shift work on chronic disease. Additional longitudinal research is clearly warranted to further evaluate the potentially complex links between shift work and LTPI, to better understand the possible modifying effects of sex and age, and ultimately, to inform strategies to support increased PA and associated reduction in chronic disease risk.