Sensor apparatus for monitoring a vital sign of a subject, comprising: at least one sensor (12, 50, 60) configured to measure a sensor signal representing a vital sign or being related to a vital sign, an evaluation unit configured to evaluate said sensor signal to determine, based on previously measured sensor signals, changes of the health condition of the subject and to identify an unsuccessful measurement or an unreliable sensor signal, and a control unit configured to control said at least one sensor to automatically change the measurement interval based on the result of said evaluation, characterised in that said control unit is configured to control said at least one sensor - to repeat measurement of the sensor signal for a predetermined period, representing a predetermined time duration or a predetermined number of times, or until the measurement is successful or the sensor signal is reliable, if an unsuccessful measurement or an unreliable sensor signal has been identified, - to issue one of the measured sensor signals, even if all measurements were unsuccessful or all measured sensor signals are unreliable; and wherein, if all measurements were unsuccessful or all measured sensor signals are unreliable, the control unit is further configured to control said at least one sensor - to repeat measurement of the sensor signal for an additional predetermined period, and - to replace the issued sensor signal by a new sensor signal measured during said additional predetermined period, if during said additional predetermined period measurement was successful or a reliable sensor signal has been measured. at least one sensor (12, 50, 60) configured to measure a sensor signal representing a vital sign or being related to a vital sign, an evaluation unit configured to evaluate said sensor signal to determine, based on previously measured sensor signals, changes of the health condition of the subject and to identify an unsuccessful measurement or an unreliable sensor signal, and a control unit configured to control said at least one sensor to automatically change the measurement interval based on the result of said evaluation, characterised in that said control unit is configured to control said at least one sensor - to repeat measurement of the sensor signal for a predetermined period, representing a predetermined time duration or a predetermined number of times, or until the measurement is successful or the sensor signal is reliable, if an unsuccessful measurement or an unreliable sensor signal has been identified, - to issue one of the measured sensor signals, even if all measurements were unsuccessful or all measured sensor signals are unreliable; and wherein, if all measurements were unsuccessful or all measured sensor signals are unreliable, the control unit is further configured to control said at least one sensor - to repeat measurement of the sensor signal for an additional predetermined period, and - to replace the issued sensor signal by a new sensor signal measured during said additional predetermined period, if during said additional predetermined period measurement was successful or a reliable sensor signal has been measured. - to repeat measurement of the sensor signal for a predetermined period, representing a predetermined time duration or a predetermined number of times, or until the measurement is successful or the sensor signal is reliable, if an unsuccessful measurement or an unreliable sensor signal has been identified, - to issue one of the measured sensor signals, even if all measurements were unsuccessful or all measured sensor signals are unreliable; and wherein, if all measurements were unsuccessful or all measured sensor signals are unreliable, the control unit is further configured to control said at least one sensor - to repeat measurement of the sensor signal for an additional predetermined period, and - to replace the issued sensor signal by a new sensor signal measured during said additional predetermined period, if during said additional predetermined period measurement was successful or a reliable sensor signal has been measured.