This study divided WCUS into four durations and examined its association with HRQoL across gender and age groups. As seen in the results part, while there are no significant results in the men group, many significant results were found in the women group. In the women group, in terms of WCUS durations, the short and the medium WCUS had generally positive effects on the responses, while the long WCUS had negative effects on the responses in general. Specifically, the short WCUS significantly improved the EQ-5D index, usual activities, and anxiety/depression domains, while the medium WCUS significantly improved the EQ-5D index and anxiety/depression domains. However, the long WCUS significantly reduced the quality of life in the self-care domain. Afterwards, when subsequently examining the associations in the women group divided by age groups, we found that the middle-aged group and the elderly group showed more significant results and had more similarity in terms of directions in WCUS effects, when compared to the results of the young group. Specifically, we found that in the middle-aged women group, the short WCUS significantly improved the EQ-5D index, while the long WCUS significantly reduced the quality of life in the self-care domain. For the elderly women group, the short WCUS significantly improved the perceived health and the usual activities domain, the medium WCUS significantly improved the EQ-5D index, the mobility and anxiety/depression domains, while the long WCUS significantly reduced the EQ-5D index and the self-care domain. Interestingly, in the young women group, both the medium and the long WCUS significantly improved the quality of life in the anxiety/depression domain.
Unlike the previous study only treating WCUS as a dichotomous variable19, this study demonstrated the inverted U-shaped associations between the duration of WCUS and HRQoL, providing new insight into the effects of WCUS durations. These findings are also aligned with previous research and sleep hygiene guidelines related to WCUS. For example, a study by Tang et al. found a U-shaped association between WCUS and adolescent self-harm behavior21, supporting our findings. In addition, from a physiological perspective, not only an excessively short WCUS is not recommended due to its lack of compensatory benefits, but also an extremely long WCUS is not desirable either as it can disrupt sleep patterns and lead to delayed sleep phase syndrome, marked by notably late bedtime and wake-up time27. Therefore, the recommended duration of WCUS (> 0 h to 2 h) identified in this study is consistent with existing physiological sleep guidelines28,29. Although it is not directly related to our research topic, we additionally performed a subgroup analysis to examine the interaction between weekday sleep duration and weekend sleep duration as a supplementary analysis (Supplementary Table 1). Interestingly, the results were generally consistent, in terms of directions of effects, with existing research14,16,19.
Next, we found that the overall effect of WCUS was significantly stronger in women than in men. These gender differences can be attributed to women’s more significant subjective discomfort with sleep and higher rates of sleep disorders than men. Previous studies have shown that women more often report poor sleep quality and efficiency, difficulty falling asleep, daytime sleepiness, and fragmented sleep1,24,25,26. In addition, a meta-analysis highlighted that women faced a 1.41-fold increased risk of insomnia compared to men for all subjects, and the ratio rose to 1.73-fold in the women group over 65 years old30. In summary, our results also indirectly support existing literature highlighting poorer sleep quality in women.
In addition, researchers attribute women’s poorer sleep quality to the pronounced variability and sensitivity of their sex hormones31. Women experience rapid and dramatic changes in female hormone secretion throughout their life cycle, including puberty, menstruation, pregnancy, childbirth, and menopause. These hormonal changes often induce physical symptoms and changes of mood that disrupt sleep. For instance, premenstrual discomfort, menopausal vasomotor symptoms, and pregnancy-related issues like frequent urination or weight gain can impair sleep quality32,33,34. Moreover, estrogen’s role, similar to cholinergic neurotransmitters and serotonin in mood regulation, may exacerbate depressive symptoms induced by its natural decline during postpartum, premenstrual, and menopausal periods35, leading to a consequent reduction of sleep quality. In addition to physiological vulnerabilities, social and environmental factors such as household chores, childcare extended into the night, unfavorable socioeconomic statuses, spousal snoring, children returning home late, and traditional gender roles have also been identified as contributing to sleep disruption in women36,37. However, despite these physiological and socio-environmental rationales, few studies have explored the association between WCUS and gender. Considering the cross-sectional characteristics of this study, there’s a need for future longitudinal sleep studies focusing on gender-specific outcomes.
In our review of WCUS studies having gender subgroup analyses, we identified only two relevant studies. Jang et al., using data from the KNHANES, investigated the association between WCUS and hyperlipidemia in 4,085 workers from 2019–202022. They found that men with 1–2 h of WCUS had a 0.64 times lower risk of hyperlipidemia than those without the sleep duration. However, this association was not significant in women. This contrast may come from the use of different outcome variables, lifestyle changes during the 2020 pandemic, and different subject criteria. Another study found associations between WCUS and subjective sleep quality with suicidal ideation and depressive symptoms in Korean adolescents23. It also reported more severe mental health symptoms in the subject with short WCUS and poorer sleep quality, but no significant gender difference was observed. The insignificant association between WCUS and gender may come from different sampling, data characteristics, and dependent variables. Further research is needed to find gender differences and the physiological backgrounds of the association.
Afterwards, we examined the relationship between WCUS and EQ-5D subdomains and found significant associations in women for three domains: self-care, usual activities, and anxiety/depression. These results are partially consistent with previous research19, which found significant effects in three domains: usual activities, anxiety/depression, and perceived health. The EQ-5D subdomains can be classified into the three categories as follows. The mobility and the self-care subdomain are into physical health, the anxiety/depression and the pain/discomfort are into mental health, and the usual activities are matched to social functioning 19,38. Given the characteristics of these subdomains, the results can be interpreted that the effect of WCUS is more pronounced for psychological variables, such as social functioning and mental health, so that the compensatory effect of WCUS may not sufficiently improve evaluations of physical health by itself.
Meanwhile, in the self-care domain, the long WCUS (> 2 h) was significantly associated with the lower quality of life than the non-WCUS. The result agrees with previous studies that identified an optimal WCUS duration for reducing the risk of metabolic syndrome and depression. Specifically, Son et al. reported that a duration between 1 h to 2 h was optimal for reducing metabolic syndrome risk12. Similarly, Kim et al. found that a duration between 1 h and 2 h was optimal for alleviating depression17. Among the subdomains related to physical health, self-care is an important component of the EQ-5D, in the sense that it evaluates one’s capability to carry out basic activities like grooming or personal hygiene. Although self-care is not typically linked to sleep disturbances, it is indirectly related to maintaining constant daily routines. Thus, if excessive WCUS disrupts these routines and sleep patterns, it may also significantly hinder self-care.
In the subsequent and more detailed subgroup analysis for women who showed significant associations, we found that patterns of the associations between WCUS and HRQoL are different according to the age groups. Overall, the short to medium WCUS significantly improved HRQoL including perceived health compared to the non-WCUS, while the long WCUS (> 2 h) was associated with worse HRQoL including perceived health. In addition, the benefits of WCUS on HRQoL were more pronounced among middle-aged and elderly women than young women in general. These results can be attributed to the age-associated decline in melatonin, which adversely affects average sleep duration, quality, and efficiency39,40. When WCUS is short or medium, it can be more beneficial to the middle-aged and the elderly groups than to the young group, because it compensates for low quality of sleep induced by low melatonin levels. However, long WCUS may significantly disrupt circadian rhythm and consequently induce reduction of sleep quality, and the groups with lower melatonin levels are more easily affected from the process40. Conversely, young women exhibited the improved quality of life in the anxiety/depression domain with the long WCUS. Although more precise physiological reasons are needed with further exploration, the benefits of long WCUS in young women seem to exceed the negative impacts of circadian rhythm disturbances, due to sufficient melatonin secretion. Transitions in a woman’s life, such as work, pregnancy, and childbirth, may indirectly influence these age-related differences. Hence, future sleep studies and interventions need to consider the age-specific sleep characteristics in women and suggest appropriate WCUS durations. However, as our results were based on a cross-sectional study, the causality was not conclusively proved, so further prospective studies are essential to clarify a causal relationship.
This study is meaningful as it offers initial insights into the associations between specific WCUS durations and HRQoL, highlighting gender differences. Findings in our study can serve as a foundation for future research to identify subjects who are more likely to benefit from WCUS for improved HRQoL (middle-aged and elderly women) and to recommend an appropriate duration of WCUS for them. In addition, the dataset used in this study is a large, representative sample from the KNHANES, which enhances the generalizability of the results.
Despite these strengths, our study has several limitations that need to be acknowledged. Firstly, there are debates in objectiveness of the self-reported sleep durations. Several studies reported they can have measurement bias41,42, while other studies have confirmed the validity of subjective sleep duration43,44. For more precise measurements, the use of actigraphy or smartwatches would be a solution, although it can be infeasible in the large-scale survey-based sleep studies16,17,19,22,23. Secondly, the large sample size of the KNHANES dataset facilitates generalization to the whole population, but the cross-sectional design of the dataset does not guarantee causal inferences. While there are some studies that identified causal effects of WCUS on neuro-mental functioning and mortality45,46, the research on quality of life remains sparse. Future studies need to employ prospective designs to detect more precise causal relationship between WCUS and HRQoL. Thirdly, the results of the subgroup analysis may need careful generalization and interpretation. The subgroup analysis can be advantageous when effects of covariates differ by gender and age group47, and this approach has been employed in many of previous sleep studies12,19,22,23,48,49. However, the approach usually focused on differences in significance between groups without providing statistical tests for comparison of effects between groups. While our subgroup analysis approach showed similar results, in terms of directions of effects, to those in the additional interaction analysis results (Supplementary Table 2), significant outcomes were reduced to a smaller number of domains, such as the EQ-5D index, self-care domain, and perceived health. Considering that the interaction analysis may need four times more sample size than analysis of main effects to achieve same power50, our results from the subgroup analysis need to be interpreted conservatively. Finally, the KNHANES dataset lacked a variety of sleep-related variables, such as sleep disorders or subjective sleep quality. Subsequent studies should consider a range of sleep-related variables, both in terms of sleep duration and qualitative factors such as sleep quality, to understand the association better.
In conclusion, our study found significant associations between WCUS and HRQoL in women. Specifically, the short or medium WCUS duration (> 0 h to ≤ 2 h) positively impacts on women’s EQ-5D index and EQ-5D subdomains (usual activities and anxiety/depression) compared to the non-WCUS. However, the WCUS exceeding 2 h was associated with the reduced quality of life in women’s self-care domain compared to the non-WCUS. Notably, the overall effect of WCUS was more pronounced in middle-aged and elderly women than in young women. These findings suggest that an appropriate WCUS duration needs to be recommended based on the gender and age of the subjects to improve their HRQOL.