The aim of our study was to characterize the dynamics of heart rate variability (HRV) during sleep in healthy subjects and in patients with obstructive sleep apnea syndrome (OSAS). Present results were compared with earlier data obtained in healthy subjects (1) performing intermittent voluntary apneas. Power spectra in low (LF) and high (HF) frequency band and non linear indices: correlation dimension (CD) and recurrence plots were computed. New indices were applied: a beat-to beat control (BBC) for the assessment of cardiovascular regulatory mechanisms as cardiac, vascular or mixed type control and COT for quantification of relative contribution of cardiac and vascular component in blood pressure variability. During Wake stage in OSAS patients mean LF component was augmented (0.035 s(2)/Hz) comparing to healthy subjects (0.012 s(2)/Hz). Nonlinear indices suggest reduced HRV dynamics complexity in OSAS patients. Similar pattern could be observed when comparing LF component, CD and recurrence parameters during spontaneous breathing and in consecutive voluntary apneas. The results correlate with 20% increase in BBC vascular control type and COT inversion form +0.08 to - 0.12. Changes in BBC and COT along with power spectra and nonlinear dynamics indices appear to signal risk and/or initiation of arterial hypertension in OSAS patients.