Daytime napping and mortality, with a special reference to cardiovascular disease: the JACC study

Background Daytime napping is associated with elevated risk of all-cause mortality in the elderly. However, the association with cardiovascular disease (CVD) risk is inconsistent. Methods From 1988 to 1990, a total of 67 129 Japanese non-workers or daytime workers (27 755 men and 39 374 women) aged 40-79 years, without a history of stroke, heart disease or cancer, completed a lifestyle questionnaire. They were followed for mortality until the end of 2003. Results During the 879 244 person-year follow-up, 9643 deaths (2852 from CVD, 3643 from cancer, 2392 from other internal causes, 738 from external causes and 18 from unspecified causes) were observed. After adjustment for possible confounders, subjects with a daytime napping habit had elevated hazard ratios (HRs) for mortality from all causes [HR 1.19, 95% confidence interval (CI) 1.14-1.24, P < 0.001], CVD (HR 1.31, 95% CI 1.22-1.42, P < 0.001), non-cardiovascular/non-cancer internal diseases (HR 1.26, 95% CI 1.16-1.37, P < 0.001) and external causes (HR 1.28, 95% CI 1.10-1.50, P = 0.001), but not for cancer death (HR 1.03, 95% CI 0.96-1.10, P = 0.400). The risk of CVD mortality associated with daytime napping was diminished among overweight subjects, but pronounced in those with weight loss after age 20 years, with non-regular employment, with lower education level and with a follow-up period < 5 years. Conclusions Daytime napping is associated with elevated risk of CVD mortality as well as non-cardiovascular/non-cancer and external deaths. Daytime napping may elevate risk of CVD death through some biological effects but, to a larger extent, some comorbid disorders causing weight loss or associated with non-regular employment and low education level could explain this association.