Relationship between Lip Seal Strength, Tongue Pressure, and Daytime Sleepiness in Japanese Workers: A Cross-Sectional Study

doi:10.21203/rs.3.rs-2858523/v1

Background: This cross-sectional study aimed to examine the relationship between lip seal strength, tongue pressure, and daytime sleepiness, among Japanese workers.

Methods: A self-administered questionnaire composed of Epworth Sleepiness Scale (ESS) was conducted on 496 workers, and excessive daytime sleepiness defined by an ESS score of 11 or over. Lip seal strength and tongue pressure were measured in all participants, and multiple regression analyses were performed to examine the effects of lip seal strength and tongue pressure on daytime sleepiness.

Results: The median ESS score was 5.0 (25th, 75th percentile: 2.0, 8.0), and 60 (12.1%) workers with excessive daytime sleepiness. The median lip-seal strength was 13.5 N (11.4, 16.3) and tongue pressure was 41.7 kPa (35.2, 48.3). Workers with excessive daytime sleepiness had significantly lower lip seal strength than did those without after adjustment for age and body mass index (BMI) (β (95% confidence interval): –0.043 [–0.081, –0.004]). However, tongue pressure was not significantly associated with excessive daytime sleepiness after adjustment for age and/or BMI.

Conclusions: These results suggest that maintaining moderate lip-seal strength may help prevent excessive daytime sleepiness in Japanese workers regardless of age or BMI.

Japanese

lip seal strength

tongue pressure

daytime sleepiness

workers

Daytime sleepiness represents one of the major occupational health issues that directly affect worker performance [1, 2]. Although the age-related depression of the root of the tongue is known to promote obstructive sleep apnea, it is thought that understanding age-related changes through dental checkups and other means could directly lead to the prevention of obstructive sleep apnea, as subjective symptoms are rare [3]. The early detection and prevention of obstructive sleep apnea are of particular interest to drivers because daytime sleepiness involve risks that directly affect the lives of the workers themselves and their customers [4]. The population of Japan is rapidly aging, which is expected to lead to increasing numbers of not only older workers, but also workers with excessive daytime sleepiness. Lee et al. [1] reported that long working times can increase the risk of obstructive sleep apnea, so increased working time for individual workers due to a decreasing working population may also lead to excessive daytime sleepiness. Maintaining a good working environment may reduce the risk of excessive daytime sleepiness and ultimately improve working performance.

Some studies have suggested associations between oral-related indicators and daytime sleepiness. Smardz et al. [5] reported finding no association between obstructive sleep apnea and sleep bruxism. Nakamura et al. [6] found that lip seal strength may be an indicator for assessing mastication and swallowing status, even in healthy young adults. Therefore, it is of great significance to examine the relationship between excessive daytime sleepiness and lip-closure force in workers across a wide age range. Although lips and tongue are composed of multiple muscles, lip seal strength and tongue pressure play an important role in articulation, swallowing, and maintaining the dentition [3, 5]. Oral health for workers has an effect on not only their prevention of chronic diseases, but also their quality at work [5]. Moreover, some effects in the oral environment have few subjective symptoms, and periodic measurement may lead to early prevention for chronic diseases. While Kugimiya et al. reported that lip seal strength has the effect by aging and was not normally distributed in adults, lip seal strength and tongue pressure were significantly correlated among adults in Japan [7] .

We have previously examined the lip seal strength and tongue pressure of Japanese workers by age group and found the possibility that (1) lip seal strength and tongue pressure decrease with age, and (2) changes in eating habits affect lip seal strength and tongue pressure in young workers [8]. Since lip seal strength and tongue pressure are closely related to eating habits, it may be possible to demonstrate the prevention of excessive daytime sleepiness by intervening in eating habits by examine the effects of lip seal strength and tongue pressure on sleepiness.

Given these backgrounds, we hypothesized that lip seal strength and tongue pressure would be associated with excessive daytime sleepiness in Japanese workers. Therefore, the present study aimed to examine the relationship between lip seal strength and tongue pressure and daytime sleepiness among Japanese workers.

2.1. Study Participants

This study was conducted from November 2021 to June 2022, and the study participants were 496 workers (454 males, 42 females) who were employed at two Japanese companies belonging to the taxi industry. Before the study began, written consent for publication was obtained from all participants. Alcohol consumption, smoking, and overweight/obesity have been reported to be associated with non-restorative sleep among healthy Japanese [7], so we administered a self-administered questionnaire composed of items on age, alcohol consumption (once a week or more, less than once a week), smoking (yes, no), weight (kg), height (cm), and obstructive sleep apnea (yes, no). Due to the COVID-19 pandemic in Japan, job descriptions are fluid, such as changing from driver to clerk, so job descriptions were not included in the questionnaire. This study was approved by the Medical Ethics Committee of Showa University School of Medicine (approval No. 21-088-A, 8 October 2021).

2.2. Outcome Variables

In this study, daytime sleepiness was set as the outcome variables. All participants were asked to answer the validated Epworth Sleepiness Scale (ESS) questionnaire, which is composed of eight items (0–3 points per item) that assess the likelihood of falling asleep during a variety of daily living situations. The Epworth sleepiness scale questionnaire (total score range, 0–24 points) was used to assess daytime sleepiness, with higher scores indicating more daytime sleepiness [9]. Normal daytime sleepiness defined by an ESS score from 0 to 10, and excessive daytime sleepiness defined by an ESS score of 11 or over (suspected obstructive sleep apnea).

2.3. Exposure Variables

We assessed lip seal strength and tongue pressure as the exposure variables in all participants. We used a lip seal strength measurement device (Lipple kun™, Shofu, Kyoto, Japan) that has been reported to be reliable [10–12]. Dental floss (30 cm long) was tied in a ring shape, and Lipple button® (Shofu) was attached to the tip of Lipple kun [12]. Lipple kun, which has an oral screen-like shape, was placed in the oral vestibule [12]. This device shown in Fig. 1.

During measurements, the subjects sat in a manner with the head position parallel to the Frankfurt plane and floor. They were also instructed about the following measurement conditions beforehand: a) resist traction using the muscle strength of the lips alone, b) do not resist traction using sucking force (do not make the intraoral pressure negative), and c) do not bend the head and body forward while resisting. The examiner stood in front of the subjects and measured the lip seal strength. After confirming that they had placed the Lipple button between the upper and lower anterior tooth regions and the button had been positioned at the center of dentition, all subjects were instructed to close their lips. Lipple kun was set at a position on the midline of the face and parallel to the floor to irradiate the subnasale with the indicator light. Then, the button was pulled slowly in the horizontal direction while maintaining the position of Lipple kun. The button was pulled until it was out of the lips, and the maximum value during this period was recorded using Lipple kun. The subjects practiced this procedure once before measurement. Measurements were performed after confirming that the subject had sufficiently understood the measurement method. Measurements were repeated three times while ensuring that a break was taken, and the mean value was defined as the lip seal strength (N).

We used a tongue pressure measuring device (TPM-01; JMS Co. Ltd., Hiroshima, Japan) to evaluate tongue pressure. An intraoral balloon-shaped probe was placed in the center of the tongue and behind the upper incisors on the hard palate. During all measurements, the participants were instructed to close their lips, press their tongue against the hard palate, and push the probe with maximum tongue force. Next, the maximum air pressure of the probe was recorded in the device. At the same time, this device displays current and maximum pressures. This device shown in Fig. 1. Tongue pressure was recorded three times, and the mean value was adopted as the participant’s tongue pressure. Each measurement was made after at least 1 minute of rest from the last measurement [13].

2.4. Statistical Analysis

To examine the effects of lip seal strength and tongue pressure on excessive daytime sleepiness, we performed multiple regression analyses. Before performing the analyses, we calculated correlation coefficients (Pearson’s r) to check for multicollinearity among the ESS score, lip seal strength, tongue pressure, age and BMI. The results were expressed as βwith 95% confidence intervals (CIs). To confirm normal contribution, a Shapiro Wilk test was performed for continuous variables. For continuous variables that are not normally distributed, median values (25th, 75th percentile) are shown instead of mean values. Multiple regression analysis was performed for continuous variables. We defined P values < 0.05 as statistically significant. This study followed the strengthening the reporting of observational studies in epidemiology (STROBE) guideline for cross-sectional study. All statistical analyses were conducted using JMP 16.2 (SAS Institute, Inc., Cary, NC, USA).

A total of 496 workers were included in the analysis. The characteristics of the participants are shown in Table 1. The results of the self-administered questionnaire indicated that 60 (12.1%) workers had excessive daytime sleepiness. Among all workers, the median lip seal strength was 13.5 N (25th, 75th percentile: 11.4, 16.3), and the median tongue pressure was 41.7 kPa (35.2, 48.3). Although 57.7% of the workers consuming alcohol, there was no significant difference between obstructive sleep apnea workers and non-obstructive sleep apnea workers (P < 0.691). Also, 35.5% of the workers smoking, there was no significant difference between obstructive sleep apnea workers and non-obstructive sleep apnea workers (P < 0.480). Among non-obstructive sleep apnea workers, the median lip seal strength was 13.3 N (11.2, 15.8), and the median tongue pressure was 41.3 kPa (35.0, 47.6). Among obstructive sleep apnea workers, the median lip seal strength was 16.5 N (13.2, 18.5), and the median tongue pressure was 46.1 kPa (38.1, 54.1). Compared with non-obstructive sleep apnea workers, obstructive sleep apnea workers had significantly higher lip seal strength (P < 0.001), tongue pressure (P < 0.001), and BMI (P < 0.001), respectively. On the other hand, there was no significant difference in alcohol consumption and smoking between obstructive sleep apnea and non-obstructive sleep apnea workers. As results of Shapiro Wilk tests, all continuous variables were confirmed not to be normally distributed.

Table 1. Characteristics of study participants.

Table 1. Characteristics of study participants.
	Total	Normal Daytime Sleepiness	Excessive Daytime Sleepiness ¹	P-value²
	(N=496)	(N=436 [87.9%])	(N=60 [12.1%])
Lip seal strength [N]	13.5 (11.4, 16.3)	13.5 (11.5, 16.3)	12.7 (10.8, 15.4)	0.028

Tongue pressure [kPa]	41.7 (35.2, 48.3)	41.8 (35.9, 48.5)	38.5 (32.5, 45.8)	0.003

ESS score (range: 0-24)	5.0 (2.0, 8.0)	4.0 (2.0, 7.0)	13.0 (12.0, 15.0)	<0.001

Age	47.0 (30.0, 55.0)	47.0 (32.0, 55.0)	44.0 (26.0, 54.0)	0.143

Male	454 (91.5)	403 (92.4)	51 (85.0)	0.053

BMI	24.1 (21.3, 27.0)	24.2 (21.5, 27.0)	23.6 (20.7, 27.1)	0.076

Overweight or Obese³	202 (40.7)	182 (41.7)	20 (33.3)	0.214

Alcohol consumption	286 (57.7)	251 (57.6)	35 (58.3)	0.911
(Once a week or more)
Smoking	176 (35.5)	158 (36.2)	18 (30.0)	0.480
(Yes)
Except where indicated n (%), values are median (25th, 75th percentile).
ESS: Epworth Sleepness Scale, BMI: Body Mass Index.
¹Defined by a ESS score of 11 or over.
²Wilcoxon rank sum test or Fisher’s Exact Test.
³Defined by a BMI of 25 or over.

Table 2 shows the correlation coefficients between the Epworth sleepiness scale scores and covariates. The correlation coefficients among the Epworth sleepiness scale scores ranged from 0.052 to 0.393, and the ESS scores significantly correlate with lip seal strength, tongue pressure, and age, respectively. Lip seal strength was correlate with tongue pressure. As all covariates were not strongly correlated with any of the other covariates, we did not exclude any covariates from the analyses to avoid multicollinearity.

Table 2. Correlation coefficients between ESS and covariates.
				Pearson's r
	a	b		c		d		e

a. ESS score		-0.131	*	-0.121	*	-0.148	*	-0.052

b. Lip seal strength				0.393	*	0.295	*	0.317	*

c. Tongue pressure						0.052		0.285	*

d. Age								0.214	*

e. BMI

OSA: Obstructive Sleep Apnoea, ESS: Epworth Sleepiness Scale, BMI: Body Mass Index.
* Statistically significant (P-value < 0.05).

Table 3 summarizes the association between ESS scores and lip seal strength according to the multiple regression analysis among all participants. In this analysis, we performed four models to examine the effects of age and BMI on lip seal strength and daytime sleepiness. Model 1 is multiple regression analysis of daytime sleepiness and oral health indicators which excluded age and BMI. Model 2 is multiple regression analysis of daytime sleepiness and oral health indicators which excluded age. Model 3 is multiple regression analysis of daytime sleepiness and oral health indicators which excluded BMI. Model 4 is multiple regression analysis of daytime sleepiness and oral health indicators. Lip seal strength was found to be positively associated with ESS scores after adjusting for sex, alcohol consumption, and smoking (β (95%CI): -0.113 (-0.223, -0.003)). However, this association was not significant after adjusting for age and/or BMI. Table 4 summarizes the association between ESS scores and tongue pressure according to the multiple regression analysis among workers. In this analysis, we performed four models to examine the effects of age and BMI on tongue pressure and daytime sleepiness. Model 1 is multiple regression analysis of daytime sleepiness and oral health indicators which excluded age and BMI. Model 2 is multiple regression analysis of daytime sleepiness and oral health indicators which excluded age. Model 3 is multiple regression analysis of daytime sleepiness and oral health indicators which excluded BMI. Model 4 is multiple regression analysis of daytime sleepiness and oral health indicators. After adjusting for sex, alcohol consumption, and smoking, tongue pressure was found to be positively associated with ESS scores (-0.043 (-0.081, -0.004)). This association remains significant after adjusting for age and/or BMI.

Table 3. Association between ESS score and lip seal strength.
	Model 1	Model 2	Model 3	Model 4
	β (95%CI)	β (95%CI)	β (95%CI)	β (95%CI)
Lip seal strength	-0.105 (-0.208, -0.002) *	-0.102 (-0.210, 0.006)	-0.078 (-0.184, 0.028)	-0.079 (-0.189, 0.031)

Age			-0.029 (-0.056, -0.001) *	-0.029 (-0.057, -0.001) *

BMI		0.008 (-0.098, 0.081)		0.004 (-0.086, 0.094)

Male	-0.973 (-1.645, -0.301) *	-0.970 (-1.643, -0.297) *	-0.861 (-1.539, -0.182) *	-0.861 (-1.541, -0.182) *

Alcohol use	0.148 (-0.220, 0.515)	0.150 (-0.218, 0.519)	0.125 (-0.242, 0.492)	0.124 (-0.245, 0.492)

Smoking	0.300 (-0.081, 0.681)	0.300 (-0.081, 0.681)	0.244 (-0.160, 0.649)	0.293 (-0.087, 0.673)

BMI: Body Mass Index.
β: Standardized partial regression coefficient
Model 1: Multiple regression analysis of ESS score and oral health indicators which excluded age and BMI.
Model 2: Multiple regression analysis of ESS score and oral health indicators which excluded age.
Model 3: Multiple regression analysis of ESS score and oral health indicators which excluded BMI.
Model 4: Multiple regression analysis of ESS score and oral health indicators.
* Statistically significant (P-value < 0.05).

Table 4. Association between ESS score and tongue pressure.
	Model 1	Model 2	Model 3	Model 4

	β (95%CI)	β (95%CI)	β (95%CI)	β (95%CI)
Tongue pressure	-0.042 (-0.079, -0.005) *	-0.041 (-0.080, -0.002) *	-0.042 (-0.083, -0.002) *	-0.043 (-0.081, -0.004) *

Age			-0.033 (-0.061, -0.004) *	-0.034 (-0.061, -0.006) *

BMI		-0.009 (-0.097, 0.080)		0.014 (-0.076, 0.104)

Male	-1.023 (-1.684, -0.361) *	-1.018 (-1.682, -0.354) *	-0.842 (-1.533, -0.150) *	-0.853 (-1.527, -0.179) *

Alcohol use	0.175 (-0.192, 0.542)	0.177 (-0.191, 0.545)	0.131 (-0.259, 0.520)	0.140 (-0.227, 0.507)

Smoking	0.313 (-0.067, 0.693)	0.313 (-0.068, 0.693)	0.249 (-0.154, 0.652)	0.296 (-0.082, 0.675)

BMI: Body Mass Index.
β: Standardized partial regression coefficient
* Statistically significant (P-value < 0.05).

Table 5 summarizes the association between lip seal strength, tongue pressure, and excessive daytime sleepiness. We performed logistic regression analyses to examine independently effects of lip seal strength and tongue pressure. Analyses of lip seal strength in tertiles (low, medium, and high) showed that workers with high or low lip seal strength were not significantly associated with excessive daytime sleepiness. Analyses of tongue pressure in tertiles (low, medium, and high) showed that workers with high or low tongue pressure were not significantly associated with excessive daytime sleepiness.

Table 5. Association between lip seal strength, tongue pressure, and excessive daytime sleepiness.
		Model 1	Model 2	Model 3

		OR (95%CI)	OR (95%CI)	OR (95%CI)

Lip-seal strength	Low	1.21 (0.63 - 2.32)		1.12 (0.58 - 2.17)
	Medium	Ref.		Ref.
	High	0.88 (0.43 - 1.79)		0.98 (0.47 - 2.02)
Tongue pressure	Low		1.50 (0.78 - 2.86)	1.47 (0.76 - 2.83)
	Medium		Ref.	Ref.
	High		0.82 (0.39 - 1.69)	0.83 (0.40 - 1.74)
All models were adjusted for age, gender, overweight or obese, alcohol use, and smoking.
Lip seal strength and tongue pressure were categorized by tertiles (Low, Medium, and High).
OSA: Obstructive Sleep Apnoea, OR: Odds Ratio, CI: Confidence Interval.

After adjusting for sex, alcohol consumption, smoking, age, and BMI, the results of the present study suggest that lower lip seal strength may lead to daytime sleepiness, but lower lip seal strength may not associate with excessive daytime sleepiness among Japanese workers. On the other hand, tongue pressure was not significantly associated with daytime sleepiness after adjusted for age and/or BMI. These results suggested that lip seal strength may be related to workers' daytime sleepiness, but there was no association for excessive daytime sleepiness.

As previous studies have suggested that daytime sleepiness related to obstructive sleep apnea may be prevented by improving the working environment [14], the results of this study further suggest that both improving oral health and examining age-related changes in lip seal strength may relate daytime sleepiness among workers. With the proportion of the older population rapidly increasing in Japan, the government is promoting the employment of the aged [15]. As the depression of the tongue root due to aging is known to promote the development of obstructive sleep apnea, examining age-related changes in lip seal strength and tongue pressure may help prevent sleep disorders among workers [3]. In particular, for drivers, the early detection and prevention of obstructive sleep apnea are key aspects of occupational health because sleep disorder involves a risk that can directly affect the lives of the workers themselves and their customers. Moreover, metabolic syndrome, a risk factor for obstructive sleep apnea, is reported to have a prevalence of about 20%, but to our knowledge, there are no reports on sex differences, severity, and effects in work systems, especially for occupational drivers and shift workers in small and medium-sized companies [11]. Therefore, it is important to measure lip seal strength to help predict obstructive sleep apnea and excessive daytime sleepiness, especially in occupations that perform life-threatening tasks, such as drivers.

Many epidemiologic studies have suggested that daytime sleepiness related to obstructive sleep apnea is a risk factor for insulin resistance [16]. Excessive daytime sleepiness has also been associated with insulin resistance in case-control studies of young, healthy, nonobese men; thus, daytime sleepiness may contribute to insulin resistance independent of age and obesity [17]. Although obstructive sleep apnea and type 2 diabetes share common the risk factors of obesity and aging, oral health may also play a role. Future studies should explore causal inferences that include eating habits, daytime sleepiness, type 2 diabetes, and indicators of oral environment including lip seal strength and tongue pressure. Lip-seal strength, tongue pressure, and eating habits can be improved with interventions and thus may lead to the prevention of both obstructive sleep apnea and type 2 diabetes.

Patients with obstructive sleep apnea have been reported to have a smaller anatomic upper airway diameter compared with healthy individuals [18, 19]. Smaller and heritable craniofacial dimensions have been reported to predispose nonobese individuals to daytime sleepiness related to obstructive sleep apnea [20]. Therefore, the possibility that weak lip seal strength may lead to daytime sleepiness is fully considered and consistent with the results of the present study.

On the other hand, measuring the oral environment over the long term is considered to be an important concept for the prevention of excessive daytime sleepiness. Sasakawa et al. [21] reported that lip seal strength may vary among healthy children depending on what they eat during meals. It is possible that lip seal strength in adulthood may be affected by eating behavior in childhood, and should therefore be examined in future studies. Future studies should also examine how lifestyle habits in childhood and other factors may affect daytime sleepiness in adulthood. As the COVID-19 pandemic has adversely affected the treatment and diagnosis of obstructive sleep apnea, Miller et al. [22] conducted a systematic review and suggested the need to manage obstructive sleep apnea using a variety of methods. The association between obstructive sleep apnea and lip closure strength found in the present study may lead to the improvement of such a management system. Another recent study reported the validity of ESS scores for assessing sleepiness in children [23]. It may therefore be possible to prevent and improve sleep disorders more effectively by focusing on ESS scores and lip seal strength from childhood.

Obstructive sleep apnea can lead to serious health problems associated with excessive daytime sleepiness and is known to be associated with morbidities such as hypertension and cerebral vascular disease [24]. Therefore, in Japan, where the working population is expected to decrease because of the increasingly low birth rate and aging population, it is urgent to secure the labor force through the prevention of or early intervention for obstructive sleep apnea. The results of this study may help establish new methods to aid in the prevention of or early intervention for obstructive sleep apnea. In this study, we decided to determine and analyze the epidemiologic profiles of workers with obstructive sleep apnea from oral health checkups because no subjective symptoms led to the possibility that many obstructive sleep apnea patients were undiagnosed.

This study had several limitations. First, because it was designed as a cross-sectional study, we were not able to examine causal relationships between sleep disorders (obstructive sleep apnea or/and daytime sleepiness) and oral health (lip seal strength or/and tongue pressure). Second, due to the small number of female workers with obstructive sleep apnea, sex differences in obstructive sleep apnea and sleep disorders could not be investigated. Some occupational health studies have indicated the presence of sex differences in the magnitude and directional specificity of the lip seal strength produced during pursing like lip closing movements in healthy young adults [25, 26]. While those studies have reported sex differences in lip seal strength, a future survey should investigate sex differences in obstructive sleep apnea and oral health. Third, the impact of the COVID-19 pandemic on workers’ sleep could not be assessed in this study. It is possible that changes in the lifestyle (especially in terms of BMI and dietary habits such as exercise, diet from the nutritional diet, and overeating) of workers due to COVID-19 prevention measures may have had an effect on sleep disorders and the measurement of oral functions [27, 28]. While previous studies have shown that obstructive sleep apnea patients with medical diagnosis have higher ESS scores than those without obstructive sleep apnea, our results suggest that there was no significant difference in ESS scores between suspected obstructive sleep apnea workers and non-OSA workers. Patients with obstructive sleep apnea may have improved sleep quality with treatment that is comparable to or better than those without obstructive sleep apnea, which needs to be investigated in future longitudinal studies. In any case, it is too difficult to diagnose obstructive sleep apnea in a large number of people during the COVID-19 pandemic, and in this study, sleep disorder was assessed by using a self-administered questionnaire, as in previous studies during a pandemic [29]. Therefore, lip seal strength and tongue pressure should be further investigated after the COVID-19 pandemic. Future studies should investigate the causal relationship between lip seal strength, tongue pressure, and sleep disorders, and the effects of the COVID-19 pandemic by conducting longitudinal studies among Japanese workers.

The results of the present study suggest that maintaining moderate lip seal strength may related to daytime sleepiness, but not related to excessive daytime sleepiness among Japanese workers. The findings also suggest that measuring lip seal strength may enable the early detection and prevention of daytime sleepiness that affect performance among workers.

Author Contributions: A.M., K.M. and Y.W planned this study. Y.I. (Yoshiaki Ihara) and K.H. contributed to improving our study in a meaningful way. A.M. drafted the manuscript. Y.I. (YoshiIhara) and H.K. performed the measurement and the data collection. Y.W. and K.H. supervised the data collection. Y.I. (Yoshiaki Ihara), K.M and A.K. supported the draft of this manuscript and data collection. A.M. contributed to the statistical analysis. Y.I. (Yoshinori Ito) and A.K. made substantial contributions to the conception of this study and the revision of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board Statement: This study was approved by the Medical Ethics Committee of the Showa University School of Medicine (approval No. 21-088-A), and approval date was Oct 8, 2021.

Consent for publication: Written consent for publication was obtained from all participants prior to participation in this study.

Ethics approval and consent to participate: This study was approved by the Medical Ethics Committee of Showa University School of Medicine (approval No. 21-088-A, 8 October 2021). Before the study began, written consent to participate was obtained from all participants.

Availability of data and materials: In this study, the data are not available in a public repository but are available from the corresponding author (Akira Minoura) upon reasonable request.

Acknowledgments: We would like to thank all the study participants. The lip-seal strength and tongue pressure measurements from this study were conducted during the COVID-19 pandemic, and participants were greatly encouraged to cooperate in preventing COVID-19 infection.

Conflicts of Interest: All authors declare that they have no conflict of interest.

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No competing interests reported.

Relationship between Lip Seal Strength, Tongue Pressure, and Daytime Sleepiness in Japanese Workers: A Cross-Sectional Study

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and Methods

2.1. Study Participants

2.2. Outcome Variables

2.3. Exposure Variables

2.4. Statistical Analysis

3. Results

4. Discussion

5. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1