Overtime work is related to nonrestorative sleep independently of short sleep time among a Japanese occupational population.

Objective: The symptoms of insomnia are defined as difficulty falling asleep, difficulty staying asleep, and early awakening. Although also a symptom of insomnia, nonrestorative sleep (NRS) is clearly more associated with objective indices than other insomnia symptoms. However, the link between NRS and overtime work duration is poorly understood.

Methods: The results of a single year's medical examinations were investigated for 26,144 Japanese office workers who were 30 to 59 years old. NRS status and lifestyle were collected through a computer-assisted medical interview. The subjects were asked about the presence or absence of NRS and their lifestyles in the most recent two to three months. The subjects were asked about their sleep times and average overtime durations per month (< 20 h/month, ≥ 20 but < 40 h/month, ≥ 40 but < 60 h/month, and ≥ 60 h/month). The relationships between NRS and overtime work duration adjusted for sleep time were also analyzed.

Results: The proportion of subjects with NRS showed a stepwise increase as overtime work hours increased. A logistic regression analysis was performed using NRS as an objective variable. The multivariate analysis demonstrated that overtime work duration (OR, 1.13; 95% CI 1.10-1.17; P < 0.001; per one-category increase) was an independent determinant of NRS.

Conclusion: For office workers, long hours of overtime work increased the NRS prevalence at any sleep duration.