This chapter discusses the interactions between obstructive sleep apnea syndrome (OSAS) and cardio- and cerebrovascular diseases, focusing on the mechanisms by which OSA may contribute to the onset and progression of cardiovascular diseases. The autonomic nervous system (ANS) is closely related to sleep from anatomical, physiological and neurochemical points of view, resulting in dynamic synchronous fluctuations in sleep phases and autonomic functions. Analysis of the circadian rhythm of heart rate variability shows that the mean high-frequency value from morning to noon is lower, whereas the mean low-frequency/high-frequency ratio is higher in OSAS patients than in controls. Measuring ANS function by means of spontaneous baroreflex cardiac modulation or other methods may not only help recognize the severity and prognostic relevance of the cardiovascular effect of OSA, but can shed light on the pathophysiological mechanisms that transform transient nocturnal alterations into sustained 24-hour derangements that require treatment.