INTRODUCTION: Subjects using opioids on a chronic basis have been reported to have a high prevalence of abnormal sleep architecture and central sleep apnea (CSA). The severity of CSA is, in part, related to blood opioid concentration. The aim of this study was to investigate subjective daytime sleepiness and daytime function in patients who are on stable methadone maintenance treatment (MMT) and to assess the possible mechanisms involving abnormal sleep architecture, CSA severity, and blood methadone concentration. METHODS: Fifty patients on MMT and 20 normal control subjects matched for age and body mass index were tested using polysomnography, blood toxicology, Epworth Sleepiness Scale (ESS), Functional Outcome of Sleep Questionnaire (FOSQ), and Beck Depression Inventory (BDI). RESULTS: The patients receiving MMT had significantly worse daytime function, were depressed, and had increased daytime sleepiness when compared with the control subjects (FOSQ 15.47 +/- 3.19 vs 19.4 +/- 0.47, BDI 14.64 +/- 10.58 vs 2.05 +/- 2.46, ESS 7.1 +/- 5 vs 2.05 +/- 1.76; all p values < 0.001). Nevertheless, daytime sleepiness in the patients receiving MMT was, on average, within the normal range (ESS < or = 10). Multiple regression analysis demonstrated that the severity of CSA, blood methadone concentration, and abnormalities in sleep architecture were not significant in predicting the variance of ESS or FOSQ (all p values > 0.05) in these patients receiving MMT. The BDI was the best predictive variable for FOSQ, explaining 16% of the variance (p = 0.004). CONCLUSIONS: Patients on stable MMT have, in general, normal subjective daytime sleepiness but impaired daytime function that partially relates to depression. The changes in sleep architecture, presence of CSA, and blood methadone concentrations do not significantly affect subjective daytime sleepiness and daytime function in these patients.