Obstructive sleep apnea syndrome (OSAS) is characterized by hypotonia of lingual and suprahyoid muscles. Genioglossus muscle is responsible for protrusion and depression of the tongue. Its dysfunction results in occlusion of the upper airways and greater incidence of apnea-hypopnea events during sleep. The aim of this prospective study was to compare the effects of daytime transcutaneous electrical stimulation of the genioglossus muscle and standard continuous positive airway pressure (CPAP) therapy on the quality of sleep, in patients with OSAS. During a 4-week study period, 19 patients with OSAS were subjected to daytime transcutaneous electrical stimulation of the genioglossus muscle before sleep and another 19 subjects underwent standard CPAP therapy. Polysomnography (apnea-hypopnea index, AHI), Epworth Sleepiness Scale (ESS) and Pittsburgh Sleep Quality Index (PSQI) were used to diagnose OSAS and to verify the efficacy of both treatments. Electrical stimulation treatment was reflected by a decrease in PSQI (p = 0.012) but did not influence ESS and AHI values (p > 0.05). In turn, CPAP therapy resulted in a significant decrease in ESS and AHI values (p < 0.001) but exerted no effect on PSQI (p = 0.089). Despite improvement of sleep quality, electrical stimulation does not seem to reduce AHI values in patients with OSAS. Daytime electrical stimulation can be considered as an adjunct treatment in OSAS. Future prospective studies should center on the identification of patients with OSAS who may benefit most from transcutaneous electrical stimulation.