Obesity has become a severe global health issue. As World Health Organization (WHO) criteria, individuals with body mass index (BMI) ≥ 30 kg/m² are considered obesity (1). WHO data shows that the global obesity population reached 890 million in 2022 (1) with this number projected to grow to 1.12 billion by 2030 (2). By 2030, almost half of American adults will grapple with obesity. Nationwide, no state will have an obesity prevalence rate lower than 35% and the rate will exceed 50% in 29 states (3). Severe obesity will afflict nearly one - fourth of women, non - Hispanic Black adults, and low-income adults (3). Obesity is a high-risk factor for numerous diseases (4, 5). In terms of the cardiovascular system (5–8), a large number of studies have confirmed that obesity can significantly increase the risk of coronary heart disease (CHD), myocardial infarction, and cerebral infarction. Obesity enhances insulin resistance (7, 9) and increases the likelihood of diabetes (6). In addition, obesity lead to obstructive sleep apnea syndrome (10), which affects the quality of sleep and causes damage to vital organs (10) such as the heart, brain, and kidneys. Moreover, obesity is closely related to the occurrence and development of diseases such as cancer (9) and osteoarthritis (11).

Factors contributing to obesity can be categorized into sociodemographic, behavioral, genetic, and living in obesogenic environment (12, 13). Researches hold that prolonged work hours (14–16) and short sleep duration (17–19) have become important factors contributing to obesity. Currently, the predominant work schedule is five workdays a week, with 8 h of work per workday. However, extended work hours are becoming increasingly common in many parts of the world. According to the information released by the International Labour Organization (ILO) (20), prior to the COVID-19 pandemic, the average number of weekly work hours for paid employment globally was approximately 43.9 h. Notably, around one-third (35.4%) of the workforce worked more than 48 h per week (20). In Japan (21) approximately 25% of Japanese companies reported that their employees, on average, worked overtime for more than 80 h per month, and a significant amount of this overtime work was not recorded (22). In South Korea, the standard work hours per week are 52 h (20). Long work hours badly invade employees’ personal lives, restricting daily activities and disrupting sleep patterns.

Existing studies have examined the relationship between work hours (15, 23) or sleep hours (19, 24) and obesity. However, there is still a shortage of research that comprehensively takes into account both work and sleep hours. Therefore, we propose a brand-new indicator, the WSR, to make up for this deficiency. In this study, both work and sleep hours are measured on a weekly basis. This paper addresses this gap through a cross-sectional study, which included 7,847 participants with aged ≥ 18 years. The value of WSR is equal to the value of work hours divided by sleep hours (WSR = work hours/sleep hours).

In this context, our study represents a pioneering effort to explore the relationship between the WSR and obesity. This is the first article to evaluate the relationship between WSR and obesity. Our research has several key findings: (1) During the period when the value of WSR continues to rise, the incidence of obesity shows an upward trend accordingly. Nevertheless, once the WSR value exceeds the crucial cutoff of 1.205, a saturation effect emerges. (2) A linear correlation was observed between work hours and the incidence of obesity. 3. A “Z” shape was observed between sleep hours and the incidence of obesity. Furthermore, the influence of WSR on obesity was found to be significantly more pronounced than that of work hours or sleep hours considered in isolation. This innovative finding provides an additional dimension for comprehending the intricate associations among work, sleep, and obesity, thereby contributing novel insights to the existing body of literature on the topic.

The interaction between work and sleep has been elaborated in existing research (26, 27). WSR introduces a novel perspective by quantifying the synergistic effect of work and sleep hours in obesity. This approach addresses the limitation of traditional single-behavior analyses (work or sleep hours alone) and fills a long-standing research gap in their interactive effects. Specifically, by normalizing work hours against sleep hours, WSR captures daily time allocation strategies, strategically guiding interventions toward dynamic balance management rather than isolated adjustments to work or sleep. It is worth noting that the impact of WSR (OR = 1.54, 95% CI 1.33–1.77, p < 0.001) on obesity is far greater than that of work (OR = 1.01, 95% CI1.01–1.01, p < 0.001) or sleep hours (OR = 0.99, 95% CI0.98 ~ 0.99, p < 0.001) alone.

Lengthy work hours are generally acknowledged as a major factor contributing to obesity (15, 28) and sleep deprivation (27). Virtanen et al. (29) analyzed 19 cohort studies included 122,078 participants and observed that long work hours (≥ 55 h/week) were associated with the risk of weight gain from normal to overweight (RR = 1.16, 95% CI 1.07–1.26), but not from overweight to obesity (RR = 1.01, 95% CI 0.89–1.14) compared with standard weekly worktime (35–40 h). Another meta-analysis (30) stressed effect of long work hours on obesity with OR = 1.23 (95% CI 1.09–1.39). In our study, we identified a linear relationship between work hours and obesity. As worktime increased, the odds of obesity also rose. The OR was 1.01 (95% CI 1.01–1.01, p < 0.001). This result is consistent with existing research findings (14, 30).

Numerous studies have examined closely the relationship between sleep duration and obesity (17–19, 31, 32). A close link between short sleep duration and obesity has been established, but there’s no consensus yet on the connection between long sleep duration and obesity. In our study, there is a negative correlation between sleep hours and obesity (OR = 0.99, 95% CI 0.98–0.99, p < 0.001) and the time period of 39 to 75 h makes a major contribution to this relationship (OR = 0.98, 95%CI 0.98–0.99, p < 0.001) with the overall data. In the model adjustment, the positive impact of sleep hours < 39 h on obesity was progressively diluted with more variables were incorporated for adjustment compared to individuals with sleep hours 39 to 75 h. In the final model, individuals with sleep hours < 39 h did not show a significant association with obesity (OR = 1.12, 95% CI 1–1.26, p = 0.056). However, the proportion of obesity was relatively higher in the group (34.64%, χ² = 22.62, p < 0.001) and p was less than 0.05 in model I/II/III. Conversely, individuals with sleep hours ≥ 75 h had a lower obesity rate (31.76%, χ² = 22.62, p < 0.001), with a protective effect against obesity (OR = 0.86, 95% CI 0.77–0.97, p = 0.011). Therefore, in our results, the relationship between sleep hours and obesity presents a “Z” shape.

In schoolchildren aged 8 to 17 years, Tambalis et al. (31) observed that 40% of these children had insufficient sleep and insufficient sleep was associated with an increased risk of overweight or obesity (OR = 1.21, 95% CI 1.17–1.25), but the relationship between long sleep duration and obesity was not mentioned. Sa et al. (32) analyzed 1,578 students from U. S. and Korean colleges and observed that obesity was associated with both short (< 7 h/night) (OR = 1.67, 95% CI 1.16–2.41, p < 0.01) and long sleep duration (> 9 h/night) (OR = 1.79,95% CI 1.03–3.43, p < 0.05), and obesity was related to short sleep among blacks only (p < 0.05) in analyses stratified by race and nationality. Jike et al. (33) analyzed 95 articles found that long sleep was significantly associated with obesity (RR = 1.08, 95% CI 1.02–1.15, p = 0.010, I² = 0%, N = 13). Maugeri et al. (34) analyzed a total of 1,482 participants aged 25 to 65 years and observed that individuals with short sleep duration (<7 h/night) have a 40% increased risk of obesity (OR = 1.40, 95%CI = 1.02–1.94, p = 0.047) and long sleep duration (>9 h/night) was associated with lower body weight (p = 0.001) and a higher proportion of underweight individuals (p = 0.002) compared to individuals with normal sleep duration. A meta-analysis of 6 articles (35) found that short sleep duration increased the risk of incident obesity in both men (OR = 1.26, 95% CI 1.13–1.40) and women (OR 1.36, 95% CI 1.16–1.59), with no significant gender differences. Another meta-analysis (36) analyzed a total of 5,172,710 participants from 153 studies suggests that short sleep not only increase the occurrence of obesity (RR = 1.38, 95%CI 1.25–1.53, p < 0.005, I² = 60%, N = 16) but is also associated with other health outcomes, such as the mortality outcome, diabetes mellitus, hypertension, cardiovascular diseases, coronary heart diseases. Zheng et al. (37) found that each hour increase in average daily sleep duration was associated with decreased odds of receiving a new diagnosis for morbid obesity (OR = 0.62, 95% CI 0.53–0.73).

The potential mechanisms linking work, sleep, and obesity were elucidated. Dietary alteration is a keystone of weight control. In fact, the actual energy intake is usually 30 to 50% higher than the intake reported by individuals themselves (38). Moreover, the longer the individuals spend working (39), the less able they are to prepare healthy foods on their own. Instead, they frequent restaurants, visit fast-food joints, or rely on food delivery services to fulfill their daily dietary needs, and as a result, the foods they consume also contain less dietary fiber (13, 29, 39, 40). Low level of physical activity is another main factor contributing to obesity (13, 41). The changes in modern work patterns, along with the predominantly office - based work model, have restricted employees’ activity time. Meanwhile, long working hours have left employees mentally and physically exhausted, limiting their time for exercise after work. This is one of the possible mechanisms by which long work hours lead to obesity (13, 42). However, not all researchers think that long work has an effect on physical activity (42). The third possible factor is sleep abnormalities caused by long work hours, including sleep deprivation or sleep disorders (27, 43). Sleep alterations also contribute to the development of obesity to a certain extent (43, 44). The changes in the microecological environment are influenced by work (45) and sleep (46, 47), and the impact of the microecology on obesity is also gradually attracting more attention (48). At the molecular level, the possible mechanisms might be attributed to changes in metabolism-related hormones, such as leptin (43, 49).

In this study, several limitations should be acknowledged. First, the lack of weighted analyses, and the exclusion of individuals who do not work or have no need to work may restrict the generalizability of the results. Second, the cross-sectional nature of the study means that it only examined the potential association between the WSR and obesity, and thus cannot establish a causal relationship. Third, the limited number of covariates included in the analysis may have influenced the interpretation of the results and introduced potential biases. Fourth, this article does not count the staff who are currently looking for jobs. Fifth, the number of working days per week and the type of work have not been mentioned, that may limit the generalizability of results to specific occupational groups. Since the number of working days per week and the daily worktime of the participants were not available, there may be some bias in the analysis.

Our findings suggest that the WSR, which combines the impact of work and sleep hours, exerts a more substantial impact on the occurrence of obesity compared to work or sleep hours alone. WSR can be used as a new reference indicator for relevant stakeholders to formulate strategies designed to promote employees’ occupational health and global health, thereby reducing the burden of obesity-related diseases.