Association of shift-work, daytime napping, and nighttime sleep with cancer incidence and cancer-caused mortality in Dongfeng-tongji cohort study

Background: Few studies investigated the combined effects of night-shift work, daytime napping, and nighttime sleep on cancer incidence and mortality. Methods: A total of 25,377 participants were included in this study. Information on sleep habits, cancer incidences, and mortalities were collected. Cox proportional hazards models were used to calculate the adjusted hazard ratios and 95% confidence intervals (HRs, 95%CIs). Results: Male subjects experienced >= 20 years of night-shift work, or without daytime napping had an increased risk of cancer, when compared with males who did not have night-shift work or napped for 1-30min [HR (95%CI)=1.27 (1.01-1.59) and 2.03 (1.01-4.13), respectively]. Nighttime sleep for >= 10h was associated with a separate 40% and 59% increased risk of cancer [HR (95%CI)=1.40 (1.04-1.88)] and cancer-caused mortality [HR (95%CI)=1.59 (1.01-2.49)] than sleep for 7-8h/night. Combined effects of three sleep habits were further identified. Male participants with at least two above risk sleep habits had a 43% increased risk of cancer [HR (95%CI)=1.43 (1.07-2.01)] and a 2.07-fold increased cancer-caused mortality [HR (95%CI)=2.07 (1.25-3.29)] than those who did not have any above risk sleep habits. However, no significant associations were observed among women. Conclusions: Long night-shift work history, without daytime napping, and long nighttime sleep duration were independently and jointly associated with higher cancer incidence among males.