Cumulative fatigue is not simply sleepiness or a bad week. Occupational health researchers define it as a progressive depletion of physical and mental resources that builds up when recovery time, chiefly sleep and rest days, consistently fails to match the demands placed on the body. It is measured here with a validated 20-item scale, with a score of 2 or above marking the threshold for the condition. Unlike acute fatigue, which resolves after a night of rest, cumulative fatigue is associated with impaired concentration, slower reaction times, elevated cardiovascular strain, and increased risk of workplace injury, a particularly serious concern for workers who spend long hours navigating traffic on motorbikes and electric scooters. Because delivery couriers operate under algorithmic time pressure, with delivery windows measured in minutes and penalties for lateness, they represent an ideal population in which to study how modern work design shapes physiological recovery.

The research team recruited 6,619 couriers across seven provinces and municipalities using a mixed sampling design, a scale that lends the results considerable weight in a workforce that is often invisible to conventional occupational health surveillance. Working hours were divided into four categories: fewer than 55 hours per week, 55 to under 70 hours, 70 to under 85 hours, and 85 hours or more. To put that in perspective, a 40-hour week is the international standard; couriers in the highest category were working more than double that, effectively spending twelve-plus hours a day, seven days a week, on the job. Sleep status was assessed on two dimensions: short sleep duration, defined as fewer than seven hours per night, and the presence of insomnia symptoms, such as difficulty falling or staying asleep. The combination allowed the researchers to disentangle the effects of sleep quantity from sleep quality.

The headline numbers are stark. After adjusting for other factors, couriers working 55 to under 70 hours per week had 54 percent higher odds of cumulative fatigue than those working under 55 hours. The odds ratio climbed to 2.10 for those working 70 to under 85 hours, and reached 4.17 for the 85-plus group. Sleep told its own story: short sleepers had nearly three times the odds of cumulative fatigue compared with those sleeping seven hours or more, while couriers reporting insomnia symptoms had roughly double the odds. These independent effects were estimated using design-adjusted logistic regression, with provinces treated as strata and delivery enterprises as clusters, a statistical approach that accounts for the fact that couriers within the same company share working conditions and are not fully independent observations.

Perhaps the most technically interesting part of the analysis lies in the dose-response curves. Using restricted cubic splines, a flexible modeling technique that allows the relationship between an exposure and an outcome to bend rather than forcing it into a straight line, the researchers found that the association between working hours and fatigue was non-linear. The risk did not rise uniformly with each additional hour; instead, the curve steepened at higher working-hour categories, suggesting a threshold effect beyond which each extra hour of work inflicts disproportionate harm. Sleep duration showed the mirror image: an inverse, predominantly monotonic association, meaning that as sleep decreased, fatigue odds climbed steadily, with the steepest increase occurring below seven hours per night. This is precisely the range in which most of the couriers in the study were sleeping, indicating that a large share of the workforce sits on the most dangerous part of the curve.

But the study’s central contribution is its analysis of joint effects. The researchers cross-classified participants into eight mutually exclusive groups based on combinations of working-hour categories and each sleep indicator, using the lowest-exposure group as the reference. The results were dramatic. Couriers working 85 or more hours per week who also slept fewer than seven hours a night had an adjusted odds ratio of 12.36 for cumulative fatigue, with a 95 percent confidence interval of 9.18 to 16.64. Those working 85-plus hours with insomnia symptoms fared only marginally better, at 9.97. These figures are not simply the sum of the independent effects; they reflect a genuine interaction between the two exposures, meaning that long hours and poor sleep amplify each other’s damage rather than merely adding to it.

The interaction analysis was conducted on two statistical scales, and the distinction matters. Multiplicative interaction, tested with product terms in the regression model, asks whether the combined odds ratio exceeds what would be expected from multiplying the individual effects. Additive interaction, quantified here by the relative excess risk due to interaction and the attributable proportion due to interaction, asks whether the combined risk exceeds the sum of the individual risks, a measure widely regarded as more relevant for public health because it reflects the actual burden of disease in a population. On the additive scale, the researchers found positive interactions for both sleep indicators: 33 percent of the fatigue risk among couriers with long hours and short sleep was attributable to the interaction itself, and the figure rose to 38 percent for long hours combined with insomnia. In plain terms, more than a third of the excess fatigue in the most exhausted workers exists only because the two exposures occur together.

The prevalence figure of 23.1 percent deserves attention in its own right. Applied to the millions of couriers employed across China’s platform delivery sector, it suggests a workforce carrying an enormous and largely unmeasured burden of exhaustion. The study was funded by China’s National Institute of Occupational Health as part of a formal program assessing the health risks of long working hours, and by a National Key Research and Development Program sub-project on preventing work-related diseases and occupational injuries, signals that the issue has reached the level of national occupational health policy. The authors point to integrated strategies that address both work-hour regulation and sleep health promotion, arguing that targeting either exposure alone would miss the synergy that drives much of the risk.

Important caveats temper these conclusions. The study is cross-sectional, capturing a single moment in time, which means it can establish association but not causation. Reverse causality is a real possibility: couriers who are already fatigued may take longer to complete their routes, extending their working hours, or may struggle to sleep because of their exhaustion rather than the other way around. The authors also note that sampling weights were unavailable, so the analysis assumed equal selection probabilities, and fatigue was self-reported rather than measured physiologically. They are explicit that the findings should be confirmed in longitudinal studies before definitive causal or policy conclusions are drawn. Still, the size of the effects, the dose-response patterns, and the consistency across both sleep indicators make the signal difficult to dismiss.

What emerges is a sobering picture of the human cost embedded in the convenience of same-day delivery. The algorithmic systems that route parcels and set delivery deadlines optimize for speed and cost, but the new data suggest they may be systematically underpricing the recovery needs of the people executing the routes. A courier working an 85-hour week is physiologically incapable of recovering on six hours of sleep a night, and the twelve-fold elevation in fatigue odds is the measurable consequence. As platform economies expand worldwide, this study provides regulators, platform companies, and public health authorities with a quantified evidence base for a conversation that has, until now, been conducted largely in anecdotes. The packages arrive on time; the question the research raises is what that punctuality is costing the people who deliver them, and whether work-hour rules and sleep health programs can be designed to pay down a debt that is currently accumulating in the bodies of an entire workforce.

Subject of Research: Associations of long working hours and poor sleep with cumulative fatigue among platform-based parcel delivery couriers in China

Article Title: Associations between working hours, sleep status, and cumulative fatigue: a cross-sectional study among platform-based parcel delivery couriers in China

Article References: Wang, J., Du, J., Wan, X., Li, S., & Sun, X. (2026). Associations between working hours, sleep status, and cumulative fatigue: a cross-sectional study among platform-based parcel delivery couriers in China. BMC Public Health. https://doi.org/10.1186/s12889-026-29788-7

Image Credits: AI Generated

DOI: 10.1186/s12889-026-29788-7

Keywords: cumulative fatigue, delivery couriers, working hours, sleep deprivation, insomnia, platform economy, occupational health, cross-sectional study, China, public health, dose-response, logistic regression