Background: It is well known that the sleeping position influences the rate of apnea-hypopnea events; however, whether events in one position may have more influence on daytime sleepiness than events in another position has not been thoroughly investigated. Objectives: We retrospectively examined the relationship between the body position-specific apnea-hypopnea index (AHI) and daily sleepiness. Methods: We assessed the sleeping body position, the body position-specific AHI and the Epworth Sleepiness Scale (ESS) in a total of 699 patients who were referred for suspected obstructive sleep apnea-hypopnea syndrome (OSAHS) and underwent diagnostic polysomnography. Results: For all subjects, only the lateral position-specific AHI (L-AHI) showed a weak but significant correlation with the ESS (r = 0.102; p < 0.05). For mild-to-moderate OSAHS patients, there was no correlation between the ESS and the AHI at any position. For severe OSAHS patients, the ESS showed a closer correlation with the L-AHI (r = 0.266; p < 0.001) than with the supine position-specific AHI (S-AHI; r = 0.141; p < 0.05). In a subgroup analysis, divided into positional and non-positional severe OSAHS patients, the correlation coefficients also identified a link between the L-AHI and the ESS. Finally, a multiple linear regression analysis indicated that the ESS was better explained by the L-AHI than by the S-AHI in severe OSAHS patients. Conclusion: The L-AHI is considered to have a stronger influence on daytime sleepiness than the S-AHI in Japanese patients with severe OSAHS.

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