Publisher Summary This chapter focuses on sleep apnea, acute changes during apneic episodes, and chronic changes with obstructive sleep apnea (OSA). Normal human sleep can be divided into rapid eye movement sleep (REM) characterized by desynchronized EEG signals, muscle atony, and dreaming, and nonrapid eye movement (NREM) sleep characterized by synchronous EEG patterns. OSA constitutes a highly prevalent sleep breathing disorder affecting an estimated 15 million adult Americans. Cardiovascular responses to OSA can be explained in part by the diving reflex, a reflex response to prolonged breath hold, during which simultaneous increase in parasympathetic activity to the heart and sympathetic activity to the periphery lead to concomitant bradycardia as well as increased peripheral arterial resistance. Given the surges of SNA occurring with each apneic episode it is not surprising that patients with OSA manifest higher blood pressure (BP) and heart rate (HR) during sleep compared to the nocturnal dip in HR and BP described in normal subjects.