Study links oral function impairments to excessive daytime sleepiness

A new study by researchers in the division of oral functional rehabilitation medicine at Showa University in Tokyo suggests that impaired oral functions, including weakened tongue strength and reduced chewing efficiency, is often associated with excessive daytime sleepiness.

The study, published in December 2025 in the journal Cureus, examined the relationship between oral function impairments and excessive daytime sleepiness.

The researchers, led by corresponding author Atsumi Sunakawa, D.D.S., Ph.D., utilized a cohort of 392 adult male participants under 65 from Japan who completed questionnaires assessing daytime sleepiness and underwent clinical evaluation of oral functions.

Sunakawa and the team looked at oral parameters such as tongue pressure, chewing performance and swallowing ability, with the same dentist — certified by the Japanese Society of Swallowing Rehabilitation — performing all measurements.

Daytime sleepiness was assessed using the Epworth Sleepiness Scale, which is a questionnaire used to examine subjective daytime sleepiness and its severity. The investigators took the results and compared oral function metrics between those who reported frequent daytime sleepiness and those with normal sleepiness levels.

They found that participants with excessive daytime sleepiness had significantly lower maximum tongue pressure and poorer chewing performance compared with those without sleepiness. Similarly, swallowing function, assessed via standardized tests, was diminished among sleepy participants.

“The pattern of decline across multiple oral function assessments suggests a robust association between oral physiology and sleep-wake regulation,” the authors wrote.

According to the researchers, the link may stem from subtle neuromuscular or anatomical abnormalities that impair airway patency, oral posture or oropharyngeal muscle function during sleep, as these are all factors that can disrupt restful sleep even in the absence of classic sleep-disordered breathing.

With that in mind, Sunakawa and the team theorized that weakened oral musculature could contribute to upper airway collapsibility or impaired airway clearance, thus triggering micro-arousals or shallow sleep that accumulate and manifest as daytime sleepiness.

The study further raises the possibility that screening for oral functional impairments might be a useful tool in sleep clinics. After all, simple, noninvasive oral assessments may help identify patients at risk of poor sleep quality or excessive sleepiness, even when standard sleep studies are unavailable or inconclusive.

That means for patients whose daytime sleepiness is not explained by sleep apnea or other sleep disorders, oral function testing could broaden diagnostic pathways and guide tailored interventions such as targeted oropharyngeal muscle training.

Although the study results are promising, the authors caution that the findings are preliminary and observational, as they did not perform overnight sleep studies or measure objective sleep architecture or respiratory events. Therefore, they cannot definitively link oral dysfunction to specific sleep disturbances.

The research team recommends larger studies incorporating polysomnography or actigraphy combined with oral function testing to clarify causal relationships and determine whether improving oral function can improve sleep quality and daytime alertness.

Still, the findings point to an underexplored domain in sleep medicine: the fact that excessive daytime sleepiness could arise from subtle oral and oropharyngeal physiologic abnormalities and not just be associated with classic sleep disorders or other lifestyle factors.

By recognizing this possibility, the authors conclude that oral function assessments deserve further investigation as part of a comprehensive sleep evaluation. If future research supports the association, interventions such as muscle-strengthening exercises, chewing training or other rehabilitative approaches might become part of a broader strategy to improve sleep quality and reduce daytime sleepiness in the future.