This research focused on the link between adult US WWI and sleep disorders. The data were analyzed through a cross-sectional study that included 46,458 participants, and the results showed a favorable correlation between WWI and sleep disorders. and this positive association was maintained after dividing WWI into quartiles (Q1-Q4). To more clearly illustrate the connection between the two, we used smoothed curve fitting and threshold effect analyses, which showed a nonlinear correlation between WWI and sleep disorders. We found the right inflection point (WWI = 11.78), where WWI to the left of the inflection point was positively associated with the likelihood of sleep disorder. Based on these results, it was demonstrated that WWI for the assessment of obesity was effective in predicting the prevalence of sleep disorders.
As far as we know, this is the first research to explore the potential connection between WWI and sleep disorders. A strong link has been shown in previous studies between obesity and sleep disturbances [17–20], A prospective cohort study spanning six years, with 1031 middle-aged and older persons, discovered a correlation between a higher body mass index and decreased sleep duration and efficiency. That is, for every unit increase in BMI, total sleep duration was shortened by 0.02 h. And this association became more obvious over time [37]. Garfield V conducted a longitudinal study of 5,015 elderly people in the UK and found that there is also a negative correlation between BMI and sleep duration. When conducting a waist circumference assessment, a very similar pattern of associations was also found. That is, for every 1 cm increase in WC at baseline, participants' mean sleep duration decreased by 0.18 minutes at follow-up [38]. Similar results have been obtained by Lallukka and Lin's research, which suggests that those with obesity have a higher likelihood of experiencing sleep disorders compared to those with average weight [18, 19]. Consistent with previous research, our study found a positive association between WWI and sleep disorders. Unlike earlier studies that did not distinguish between different types of obesity measurements, our study specifically highlights the relevance of WWI.
Despite accumulating evidence confirming the association between obesity and sleep, the paradox of obesity endures [39, 40]. The fundamental cause of this is the shortcomings of conventional indicators, such as BMI, which is limited to measuring the amount of fat and is not sensitive to measuring fat distribution, especially when the BMI is < 30 kg/m2 [41], and is susceptible to age, gender, and ethnicity factors [22]. WC is a measure of the degree of abdominal obesity but has a strong correlation with BMI. Therefore, WC is limited as an independent predictor of BMI [24]. Zhang et al. analyzed data from 11,155 community members and discovered that the distribution of body fat was a major predictor and cause of obstructive sleep apnea (OSA), regardless of height and weight [42]. Thus, accurately reflecting body fat distribution is particularly important in predicting sleep disorders. WWI is a novel obesity metric that combines the benefits of WC with a weaker relationship to BMI, offers a more accurate and complete response to central obesity, independent of body weight, as compared to established obesity indicators [25]. WWI was shown to be adversely correlated with all measures of muscle mass and favorably correlated with all measures of fat mass in cross-sectional research including 602 Korean adults [43]. This finding was equally applicable to different races and populations [44], demonstrating the stability and reliability of WWI. In addition, the index is computationally simple, cost-effective, and has obvious advantages in disease prediction. Therefore, it deserves adequate attention from healthcare professionals.
Subgroup analysis in this study found gender differences in the incidence of WWI and sleep disorders, with a higher probability of sleep disorders in the male group. This is more similar to the studies by Zhou [45] and Nishiura [46] et al. However, it has also been suggested that females are more likely to develop sleep disorders [47, 48]. The reason for this may be due to methodological differences such as study design or exposure assessment [47]. It may also be related to hormonal or gender roles and sociological factors [49]. In a subgroup analysis of age, we found that for those under 60 years of age, the prevalence of sleep disorders was higher for each unit increase in WWI. This may be related to the fact that the young and middle-aged population is at a bottleneck in their career ascent and suffers from greater physical and mental stress and environmental factors. In terms of race, non-Hispanic blacks had a higher prevalence of obesity compared to other races. This more closely resembles the findings of Hales et al. [50]. Compared to individuals who engaged in physical activity, those who did not exercise had a higher likelihood of experiencing sleep disorders. This is more in line with what Eid et al. found [51]. This might be explained by the fact that engaging in physical exercise increases daily energy expenditure, a significant protective factor against obesity [52].
The association between obesity and sleep disorders could be explained by a number of different mechanisms.First, there is an interaction between obesity and sleep, and obesity may have a direct effect on sleep quality [49], which increases the risk of sleep apnea and other sleep disorders, which in turn impairs sleep quality [53]. Fat individuals with a higher BMI or waist circumference may have a higher visceral fat ratio than thin individuals and are prone to airway collapse or obstructive disorders during sleep, making them susceptible to snoring or sleep apnea [54]. This, in turn, can lead to difficulties in maintaining sleep, with shorter and more fragmented durations [53]. Secondly, obesity predisposes to an increased risk of upper airway obstruction by causing fatty deposits on the tongue, neck, and lungs, leading to narrowing of the upper airways, impaired breathing, and reduced lung capacity [55–57]. And obesity-induced leptin resistance, or relative leptin deficiency, is also likely to be associated with diminished respiratory drive and hypercapnia [58]. In addition, elevated levels of pro-inflammatory factors or hypothalamic-pituitary-adrenal axis dysfunction in the body may also contribute to symptoms such as daytime sleepiness in obese patients [59]. Finally, sleep disorders can also have an impact on obesity. Sleep deprivation, both in terms of quality and duration of sleep, has been associated with difficulty controlling appetite, which can lead to obesity [56].
Our study has several significant advantages. First, it is based on the NHANES database with a large and representative sample size. Second, we performed subgroup analyses to elucidate further WWI association with sleep disorders in different populations. Finally, we also adjusted for confounding factors to obtain more reliable results. Our study does, however, have several shortcomings. First off, we are unable to prove a causal relationship between WWI and sleep disorders because this is a cross-sectional research. Second, we relied on patients' self-reports when performing sleep disorder assessments, did not typify sleep disorders, and did not consider the interference of medications with sleep disorders. Furthermore, although we have included many confounding factors, we are unable to totally exclude the impact of other variables. Finally, we have conducted a survey of the US population, and therefore, the applicability to other countries needs to be further confirmed.