60-year-old morbidly obese man becomes difficult to arouse 12 hours after undergoing elective right knee replacement. He has a history of osteoarthritis, hypertension, and sleep apnea. Outpatient medications are acetaminophen and hydrochlorothiazide. He also uses nocturnal bilevel positive airway pressure ventilation. His surgery was uncomplicated, and he has received regularly scheduled doses of intravenous morphine sulfate for pain. On physical examination, temperature is 36.6 °C (97.8 °F), heart rate is 80/min, respiration rate is 10/min and shallow, and blood pressure is 130/85 mm lg. Cardiac examination is normal and unchanged from his examination on admission. Pulmonary examination reveals distant breath sounds without wheezes or cradles. Ile is moderately responsive to sternal rub, snoring loudly. Oxygen saturation is 90% by pulse oximetry with the patient breathing oxygen, 2 L/min, by nasal cannula. The pupils are equal, round, and reactive to light. Neurologic examination is normal. Which of the following diagnostic studies is most likely to determine the cause of this patient's diminished level of consciousness?