Although daytime sleepiness is commonly associated with obstructive sleep apnoea (OSA), the relationship between OSA severity and subjective sleepiness has been documented elusive. This study aimed to identify clinical and polysomnographic determinants of subjective sleepiness among patients suspected of having OSA. A sleep clinic-based sample of 915 patients was interviewed with a structured questionnaire and underwent diagnostic overnight polysomnography. Subjective sleepiness was quantified by Epworth Sleepiness Scale (ESS). Excessive daytime sleepiness (defined as ESS score > 10) was present in 38.8% of patients. In multiple linear regression analysis, respiratory disturbance index [RDI; used to define (whenever RDI was >5) and quantify OSA], depression and diabetes were the most important determinants of ESS score accounting for 17%, 11% and 6% of its variability respectively. Chronic obstructive pulmonary disease (COPD), stroke, heart disease, alcohol use and body mass index were less important determinants of ESS score explaining 1–3% of its variability. In conclusion, OSA should not be considered the sole potential cause of increased subjective sleepiness in patients suspected of having OSA. Primarily depression and diabetes, but also COPD, stroke, heart disease, alcohol use and increased body mass index may contribute to increased subjective sleepiness.