Background: More than half of OSA patients do not present daytime sleepiness (EDS); however underling potential mechanisms are not entirely clear.

Aim: To investigate whether PSG data differ between non-sleepy and sleepy OSA patients within the same disease severity at the time of diagnosis as a potential cause of presence or absence of EDS.

Methods:1,104 out of 4,242 OSA patients, without comorbidities (AHI?5) were included. Based on AHI, patients were classified into mild (n=206, 19%), moderate (n=318, 29%) and severe OSA (n=580, 53%) groups. We subsequently divided these groups based on Epworth Sleepiness Scale (ESS) into two subgroups: sleepy (ESS>10) and non-sleepy (ESS ? 10). PSG data were compared between groups and multivariable logistic regression was used to identify differences after adjustment for age, gender and BMI.

Results: Mild OSA sleepy patients exhibited higher WASO (p=0.03) and TST90 (p=0.01) compared to non-sleepy patients. Decreased sleep latency (p=0.02), mean (p=0.04) and lowest SaO2 (p=0.01) was noted in sleepy moderate OSA patients. In severe OSA sleepy subgroup SWS sleep (p=0.002) and sleep latency were lower (p<0.001) whereas arousal Index (p=0.001) was higher. Additionally, indices of OSA severity (AHI, ODI, mean and lowest SaO2, TST90, p<0.001 respectively) were worse in sleepy severe OSA patients. These associations persisted after adjustment for age, gender and BMI.

Conclusions: Our results suggest that parameters of sleep architecture and indices of OSA severity may account for the presence or absence of EDS in OSA patients and could be useful for illustrating underling pathophysiological mechanisms towards precision medicine.