Fatigue among Employees Who Work Over the Karoshi Line (More Than 80 Hours Overtime per Month): Hybrid Approach Based on Experimental and Field Studies

Introduction: Empirical data regarding the adverse effects of overtime have been gathered. However, little is known about the magnitude of overtime risk, although the existence of risk has been discussed. Notably, it is informative to understand the risk magnitude in specific situations in terms of prevention. This study was intended to illustrate the risk magnitude by comparing a 40-hour sleep deprivation experimental study and field study targeted at employees with long working hours.

Methods: This study was based on a hybrid approach with experimental and field studies. In the 4- day experiment, we conducted a 40-hour sleep deprivation study to gain the reference value regarding excessive fatigue situations. Nine individuals (44.4 ± 6.0 years, including 4 females) participated in this experiment. The participants were required to stay awake in a sleep laboratory during a 40-hour period (from 7:00 am on the 2nd day to 11:00 pm on the 3rd day). A 40-minute drive simulator task (Euro Truck Simulator 2) was performed hourly during the sleep deprivation period. Additionally, 100 employees (44.7 ± 9.1yr, including 18 females) participated in the 10-day field observational study. We focused on 7 individuals who worked more than 80 overtime hours (i.e., Karoshi line) in the pre-survey. In both studies, the participants were required to perform a 3-minute psychomotor vigilance task (PVT-B) using their smartphone. Measurements were taken hourly in the experimental study, whereas measurements were performed twice daily (morning, night) in the field study. We focused on the mean and 95% CI regarding median reaction times (RTs) as parameters to avoid a response time deviation.

Results: In the 40-hour sleep deprivation setting, the mean and 95% CI of median RT was 369.8 millisecondsec (321.4−418.3 millisecondsec). On the other hand, the mean of median RT in the field study ranged from 289.3 to 482.9 millisecondsec. The longest median RT was observed on Saturday morning, and the shortest was on Wednesday night. RTs among employees who worked more than 80 overtime hours per month were suggested to be equivalent to those who experienced 40-hour sleep deprivation. The tendency was obvious from Thursday to Saturday.

Conclusion: Our primary findings suggest that fatigue from 40 hours of sleep deprivation could reach the level of fatigue among employees who worked over the Karoshi line. Applying the findings to specific workers who are required to sustain attention to prevent serious accidents (e.g., truck drivers, nurses, and doctors) could be beneficial in understanding the magnitude of long working hours. Support: This study was supported by the National Institute of Occupational Safety and Health, Japan (N-P05–01).

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Artikel online veröffentlicht:
08. Oktober 2025