Obstructive Sleep Apnea Syndrome (OSAS) is characterized by repeated episodes of airway obstruction while sleeping. Upper airway obstruction while sleeping leads to a dramatic decrease in oxygen saturation and to hypoxemia finally, in which consequence the patient rapidly awake. Clinical signs are sleepiness and functional cognitive deficits. The Gold standard therapy is "continuous positive airway pressure" ventilation during sleep. However, the success depends strongly to the patient´s compliance. Surgical treatment is an alternative option, which could be considered if clinical success failed. A comparison of the cognitive function of both therapies is not yet analyzed. The study addresses the question which of this treatment options is favorable regarding cognitive function and outcome.