The longitudinal relationship between night-time sleep duration, midday napping, and frailty among middle-aged and older people in China: a prospective analysis

Frailty is an important issue presented by ageing. Night-time sleep and midday napping are important modifiable factors influencing health, but their impacts on frailty remain unclear. We used five waves of data from the China Health and Retirement Longitudinal Study (2011-20), with 15 333 participants in the baseline sample. We used fixed effects regression models to explore longitudinal relationships between night-time sleep duration, midday napping, and frailty index (FI). We added interaction terms of sleeping and napping to the regression model to explore their combined effects. We further used the Cox proportional regression model to quantify risks for frailty. Compared to sleeping seven to nine hours, sleeping <6 hours (FI = 0.016), six to seven hours (FI = 0.004), and >9 hours (FI = 0.005) were significantly associated with a mean increase in FI separately. Napping >90 minutes significantly increased FI by 0.003 compared to non-nappers. Effects of sleeping six to seven hours and >9 hours on frailty were separately enhanced by napping >90 minutes and any napping duration (except 60-90 minutes). Sleeping <6 hours and six to seven hours increased frailty risk by 44% (hazard ratio (HR) = 1.44) and 12% (HR = 1.12), respectively. Frailty risk was increased by napping >90 minutes by 14% (HR = 1.14) compared to non-nappers. Short (<7 hours) or long (>9 hours) sleep and prolonged midday napping (>90 minutes) were associated with frailty among the Chinese middle-aged and older population. The compensation effect of napping for short night-time sleep was not found in this study, and certain napping durations even increased risks of sleeping six to seven hours and >9 hours for frailty.