Men who have worked night shifts for more than 20 years, or who work night shifts without daytime napping, or sleep for more than ten hours per night on average may have an increased risk of cancer, according to a study published in Annals of Medicine.
The study, led by scientists based at the Huazhong University of Science and Technology, Wuhan, China, reviewed data obtained via interviews with middle-aged and older Chinese in the Dongfeng-Tongji Cohort Study, a cohort of approximately 27,000 retired workers from the Dongfeng Motor Corporation.
The researchers sought to investigate the independent and combined effects of three sleep habits on cancer incidence; night shift work, daytime napping, and night time sleep. Via a questionnaire they ascertained individuals who had worked night shifts for over 20 years, had a habit of taking day time naps, and when they usually went to sleep at night and woke up in the morning.
The authors report that men who had worked night shifts for over 20 years had a 27% increased risk of cancer incidence, and that men that did not nap in the day time had double the risk of cancer of those who took a 1 to 30-minute nap. They also found that men who slept for more than ten hours per night had an increased risk of cancer. However, no such relationship was observed in women.
The researchers also found that male participants with at least two of these sleep habits (long-term night shift work, lack of daytime napping, or sleeping over ten hours per night) had a 43% increased risk of cancer incidence and a two-fold increase in cancer mortality compared to those who exhibited none of the sleep habits.
The authors recognise that their conclusions may be limited by the self-reported lifestyle data, the ageing nature of the cohort and a relatively short-term follow-up period. They recommend that a longer term study should follow up this initial study to verify their findings.