<qa>  What is the main purpuse to evaluate the SOMNOtouch system?To measure the spontaneous baroreflex sensitivity in ambulatory patient, a device made for measuring blood pressure beat by beat and continuously, the SOMNOtouch system, could be a useful tool. The SOMNOtouch system has never been used to assess the sensitivity of the baroreflex. A good correlation for measuring the baroreflex sensitivity between the system SOMNOtouch and digital photoplethysmography as a reference method is expected but should be evaluated. The main purpuse is to evaluate the SOMNOtouch system to quantify the spontaneous baroreflex sensitivity in healthy young subjects, in elderly healthy subjects and in subjects with heart failure. We will also evaluate the feasibility of measuring the baroreflex sensitivity in ambulatory conditions for 24 hours, the variations in sensitivity of the baroreflex sensitivity between day and night, the reproducibility of orthostatic tolerance test. 