Daytime napping and increased risk of incident respiratory diseases : symptom, marker or risk factor?

Background: We have identified a strong association between daytime napping and increased mortality risk from respiratory diseases, but little is known about the relationship between daytime napping and respiratory morbidity. Methods: Data were drawn from the European Prospective Investigation into Cancer and Nutrition (EPIC)-Norfolk cohort. Participants reported napping habits during 1998-2000, and were followed up for respiratory disease hospital admissions until March 2009. Cox proportional hazards regression was used to examine the association between daytime napping and respiratory disease incidence risk. Results: The study sample included 10978 men and women with a mean age of 61.9 years old, and a total of 946 incident respiratory disease cases were recorded. After adjustment for age, sex, social class, education, marital status, employment status, nightshift work, body mass index (BMI), physical activity, smoking, alcohol intake, self-reported general health, hypnotic drug use, habitual sleep duration and pre-existing health conditions, daytime napping was associated with an increase in the overall respiratory incidence risk [Hazard Ratio (HR) =1.32, 95% confidence interval (CI) 1.15, 1.52 for napping<1h; HR=1.54, 95% CI 1.14, 2.09 for napping≥1h]. This association was more pronounced for lower respiratory diseases, especially for the risk of chronic lower respiratory diseases (HR = 1.52, 95% CI: 1.18, 1.96 for napping