The purpose of this study was to clarify the relationship between chronotype and self-rated health in a large sample of Japanese daytime employees.
Chronotype, which is the natural inclination or preference of your body to sleep at a certain time, has been implicated to be associated with various health issues including sleep problems and depression. In general, chronotype is understood as being an “early bird” versus a “night owl.” Some studies have proposed that chronotype is associated with health status, however, not much studies have specifically focused on perceived health and its association with chronotype.
A total of 65,941 non-shift daytime employees (48,932 men and 17,009 women), aged 18-70 (mean 45) years, in 227 different organizations representing various industries and occupations were surveyed by means of a self-administrated questionnaire between 2007 and 2012. In the questionnaire, participants answered questions regarding bedtime and wake-up times on weekdays (workday) and weekends (non-workday) as well as self-rated health. Chronotype was derived from mid-sleep time on weekends corrected for sleep debt on weekdays (MSFsc). Chronotype was arbitrary categorized as a) early-type: MSFsc <3:00 am (reference group; early bird), b) intermediate-type: MSFsc 3:00-5:00 am, and c) late-type (night owl): MSFsc >5:00 am as recommended by Ronnenberg et al. Self-rated health was asked by a following question: How would you describe your current health status? Response options were: (1) very good, (2) quite good, (3) quite poor, or (4) very poor. Multivariable logistic regression was used to examine the association between chronotype and self-rated health with adjustment for potential confounders (age, sex, smoking, alcohol consumption, exercise, body mass index, year of survey, season, social support, quantitative workload, job control, overtime, job type, industry type, number of chronic disorders, daily sleep durations, and insomnia symptoms).
Regarding chronotype, 26.0 % were categorized as early-type, 59.7% were intermediate-type, and 14.4% were late-type in this population. Those who responded poor health was 27.4% in this population. Later chronotype was significantly associated with an increased likelihood of poor self-rated health. The multivariable adjusted odds ratio (aOR) with 95% confidence intervals for intermediate-type and late-type for poor self-rated health was 1.07 (1.02-1.13) and 1.23 (1.15-1.32), compared to those with early-type (reference group) of chronotype, respectively.
Past studies have suggested that short sleep or poor sleep is a risk factor for poor self-rated health. However, the results of this study implies that those with later chronotype could be a potential risk factor as well. Thus, controlling chronotype may contribute to improve health status among active employees. In this large population-based cross-sectional study, employees with later chronotype had a significantly higher odds for poor self-rated health in a dose-dependent manner. The study suggests that poor self-rated health is not only associated with poor sleep or short sleep duration as reported in past studies, but also with the chronotype. The results raise the importance to provide sleep training program aiming to enhance health status.
Chronotype is associated with poor self-rated health among relatively health employees.