What are the causes of excessive daytime sleepiness in patients with sleep-disordered breathing?

Recently, the European Respiratory Journal published a study by MEDIANO et al. [1] on the determinants of excessive daytime sleepiness (EDS) in obstructive sleep apnoea (OSA) patients. The study did not find differences in body mass index (BMI), age or apnoea/hypopnoea index (AHI) between OSA patients with and without EDS, but found significantly more pronounced nocturnal hypoxaemia in patients with EDS, and concluded that nocturnal oxygen desaturation is involved in the pathogenesis of EDS. In his letter commenting on this work, BAHAMMAM [2] pointed out that the possible role of obesity hypoventilation syndrome (OHS), a known cause of EDS, was not ruled out by MEDIANO et al. [1]. OHS patients have the same AHI of pure OSA patients matched by age and BMI, but are characterised by apnoeas of longer duration and more severe nocturnal hypoxaemia [3], like the EDS patients studied by MEDIANO et al. [1]. In other words, BAHAMMAM [2] suggests that the lower nocturnal oxygen saturation and the associated daytime somnolence observed by MEDIANO et al. [1] in the EDS group might be due to OHS. Thus, BAHAMMAM [2] also concludes that low nocturnal blood oxygen saturation has a fundamental role in EDS, whether or not it is related to OHS.