JMIR Publications

ABSTRACT

Background:

Chronotype is the propensity for a person to sleep at a particular time during 24-hour. It is largely regulated by the circadian clock but constrained by work obligations to specific sleep schedules. The discrepancy between biological and social time can be described as social jetlag (SJL), which is highly prevalent in modern society and associated with health problems. SJL and chronotypes have been widely studied in West countries, but never been described in China.

Objective:

We characterized the chronotypes and SJL in mainland China objectively by analyzing a database of Chinese sleep-wake pattern recorded by up-to-date wearable devices.

Methods:

We analyzed 71176 anonymous Chinese who were continuously recorded by wearable devices at least for one week between April and July in 2017. The chronotypes were assessed in 49573 subjects by the adjusted mid-point of sleep on free days (MSFsc). Early, intermediate and late chronotypes were defined by arbitrary cut-offs of MSFsc< 3 h, between 3 h and 5 h, and > 5 h. In all subjects, SJL was calculated as the difference between mid-points of sleep on free days and work days. The correlations between SJL and age/ body mass index (BMI)/MSFsc were assessed by Pearson’s correlation. Random forest was used to characterize which factors (age, BMI, sex, nocturnal and daytime sleep durations and exercise) mostly contribute to SJL and MSFsc.

Results:

The mean total sleep durations of Chinese is about 7-hour, with females sleep on average 17 minutes longer than males. People taking longer naps sleep less during the night, but they have longer total 24-hour sleep durations. MSFsc follows a normal distribution, and the percentages of early, intermediate, and late chronotypes are approximately 26.76% (13266/49573), 58.59% (29045/49573), and 14.64% (7257/49573). Adolescents are later types compared to adults. Age is the most important predictor of MSFsc suggested by our random forest model (relative feature importance: 0.772), and no gender differences are found in chronotypes. SJL follows a normal distribution and 17.07% (12151/71176) of Chinese have SJL larger than 1-hour. 31.53% (22442/71176) Chinese have SJL<0. 53.72% (7127/13266), 25.46% (7396/29045) and 12.71% (922/7257) of the early, intermediate and late chronotypes have SJL<0, respectively. SJL correlates with MSFsc (r=0.54, P<.001), but not with BMI (r=0.004, P=.297). Random forest model suggests that age, nocturnal sleep and daytime nap durations are the features contributing to SJL (their relative feature importance is 0.441, 0.349 and 0.204, respectively).

Conclusions:

Our data suggest a higher proportion of early compared to late chronotypes in Chinese. Chinese have smaller SJL than the results reported in European populations, and more than half of the early chronotypes have negative SJL. In Chinese population SJL does not associated with BMI. People of later chronotypes and long sleepers suffer more from SJL.