In this seven-year longitudinal cohort study of 5,262 subjects, we identified four different nighttime sleep trajectories and three different daytime sleep trajectories in Chinese adults over 45 years. We found that nighttime sleep trajectories were significantly correlated with the occurrence of multimorbidity and daytime napping cannot compensate for the risk of insufficient night sleep. We also found a correlation of baseline nighttime sleep duration and daytime napping duration with the occurrence of multimorbidity, which supports conclusion of traditional studies.
Our study found that participants with persistent short nighttime sleep duration had a higher risk of multimorbidity. While exploring the relationship between the baseline nighttime sleep duration and the incidence of multimorbidity, we found that participants with a baseline nighttime sleep duration of ≤4 hours had an increased the risk of developing multimorbidity compared with those with a baseline nighttime sleep duration of 7-9 hours. The dose-response curve showed a U-shaped relationship between baseline nighttime sleep duration and the risk of multimorbidity. At present, few studies have linked nighttime sleep duration to risk of multimorbidity, but some studies have found an association between nighttime sleep patterns and certain diseases. For example, a study found that people with consistently high nighttime sleep efficiency had lower prevalence of hypertension, circulatory system problems, arthritis, respiratory problems, and depression than those with consistently low nighttime efficiency[26]. Sustained short sleep duration and sleep variability in adulthood were associated with increased risk of type 2 diabetes later in life[27]. Another study found the association of persistent poor sleep quality and persistent sleep problems with continued deterioration in physical health[28]. Other studies have identified associations between persistently short nighttime sleep duration and the prevalence of hypertension[29], the risk of diabetes[27], and the risk of a first cardiovascular event[17]. In addition, the Whitehall II study and a Korean study respectively found an association of consistently short sleep duration with the risk of dementia and depression[18, 19]. Several cross-sectional studies[10, 11, 30] and a longitudinal cohort study[14] supported our findings that short baseline nighttime sleep increases the risk of multimorbidity.
We were the first to examined the association between daytime napping duration trajectories and the occurrence of multimorbidity. We found that people with persistent short or long duration of daytime naps had an increased the risk of multimorbidity, but unfortunately this association was not statistically significance. In addition, sensitivity analysis indicated that compared with no naps, those who took 0~1 hour daytime naps increased the risk of multimorbidity in people aged 60 years and above. So far, there are few studies on daytime nap patterns and multimorbidity, but a recent study found that individuals with the high-concave daytime napping duration pattern had a higher risk of hypertension, compared with individuals with a low-stable napping duration pattern[20]. Past studies have found the association of baseline daytime napping duration with metabolic syndrome[31], stroke[32], heart failure[33]. In contrast, a meta-analysis of cohort studies only found an association between daytime napping duration and cardiovascular in women crowds[34]. These studies support our findings to some extent that longer daytime nap may be associated with adverse outcomes.
Our study also found that compared with those with persistent normal nighttime duration and those with persistent seldom daytime napping duration, other sleep trajectory combinations increased the risk of developing multimorbidity to varying degrees, especially in those with persistent short nighttime sleep duration and persistent short daytime napping duration. Only a few studies have examined association between the combined effects of a single measure of nighttime and daytime sleep duration and adverse outcomes. For example, studies found that people who slept less at night and take longer naps had an increased risk of diabetes[35] and stroke [36]. Another study found that shorter total sleep duration combining with longer naps were associated with higher mortality in colorectal cancer survivors[37].
Several potential mechanisms may explain the association between reduced nighttime sleep duration and increased daytime napping duration and the risk of multimorbidity. Experimental evidence suggests that sleep is involved in inflammation, immune response, glucose regulation, and energy balance. It has been demonstrated that inflammation is one of the important pathogenesis of multimorbidity[38, 39]. Chronic sleep deprivation leads to chronic systemic low-level inflammation, whose persistence has been linked to obesity, cardiovascular disease, asthma, chronic pain, cancer, and neurodegenerative diseases[40]。A meta-study has found that napping itself is not harmful, and that inflammation is the mediator linking napping to later poor health[41]. After sleep restriction, napping or sleep extension could improve the recovery processes of alertness and immune and inflammatory parameters[42]. However, daytime napping appears to have limited compensation for the risk of nightime sleep deprivation[41, 43]. Thus, consistent with previous studies, we found that daytime napping may have a protective effect on short-night sleepers, but only in a limited way.
Strengths and Limitations
The advantage of this study is that subjects were selected from a nationally representative longitudinal cohort. In this study, we assessed the longitudinal daytime napping habits and nighttime sleep habits and explored the association between longitudinal sleep habits and occurrence of the multimorbidity. We also examined the combined effects of nighttime sleep habits and daytime napping habits on multimorbidity. Potential confounding factors were adjusted, such as sex, age, marital status, education, smoking, alcohol consumption, and financial support.
The study has several notable limitations. First, using self-reported sleep duration took place of objective measures, which may create bias. Second, there is a lack of measurements of other latitude sleep variables, such as sleep apnea, insomnia, and sleep quality. Moreover, the association between sleep duration trajectories and the occurrence of multimorbidity need to be validified by a study with longer time follow-up and larger sample size. Finally, although multiple potential covariables were adjusted, other confounding could not be ruled out.