A method of monitoring an estimation of an arousal component of heart rate (HR PhAr) of an individual human or animal on the basis of a model of a bioprocess of the human or animal, the model being capable of integrating measured, real-time information on one or more bioprocess inputs and one or more bioprocess outputs and linking this to the estimation of the arousal component of the heart rate (HR PhAr) of the individual human or animal, the heart rate (HR tot) having a physical component (HR phys) and the arousal component (HR PhAr), the method comprising - generating the model of the bioprocess on-line, wherein model parameters are estimated and updated in real-time based on on-line measurement of bioprocess inputs and bioprocess outputs, - inputting in the model the real-time information on the bioprocess including bioprocess inputs and bioprocess outputs, - generating model outputs responsive to the real-time information on the bioprocess inputs using a dynamic and adaptive data-based on-line modelling technique, wherein at least one of the bioprocess inputs used as model input, is physical activity of the human or animal and at least one of the model outputs is an estimated component of the heart rate (HR mech), being a bioprocess output relating to the physical component of the heart rate, based on the physical activity, and - measuring the heart rate (HR tot) of the individual human or animal, for obtaining a measured heart rate (HR tot), - further comprising the step of using a value of an unknown variable to monitor and/or control the arousal component of the heart rate (HR PhAr), wherein the value of the unknown variable is determined by subtracting in real-time the estimated component of the heart rate (HR mech), a component of the heart rate relating to basal metabolism (HR bmr) and a component of the heart rate relating to heat balance (HR heat) from the measured heart rate (HR tot) and the resulting difference is the value of the unknown variable attributed to the arousal component of the heart rate (HR PhAr); wherein H R tot = H R phys + H R PhAr with H R phys = H R mech + H R bmr + H R heat wherein generating the model on-line includes expressing the heart rate (HR tot) as y t = B q F q u t − nk + C q D q e t in which the physical component of the heart rate (HR phys), being the estimated component of the heart rate (HR mech) with a time delay nk, the component of the heart rate relating to basal metabolism (HR bmr) and the component of the heart rate relating to heat balance (HR heat), are expressed as B q F q u t − nk and in which the error is the resulting difference corresponding to the value of the unknown variable attributed to the arousal component of the heart rate (HR PhAr) expressed as C q D q e t wherein - y(t) is a time series of the real-time measured heart rate (HR tot); - t is time; - u(t-nk) is a time series of the bioprocess input, that is the physical activity of the human or animal, used as model input; - nk is a time delay between the bioprocess input and the bioprocess output; - e(t) is a dynamic error term describing the unknown variable attributed to the estimation of the arousal component of the heart rate (HR PhAr); - B(q), F(q), C(q) and D(q) are polynomials; - q is a backward shift operator. - generating the model of the bioprocess on-line, wherein model parameters are estimated and updated in real-time based on on-line measurement of bioprocess inputs and bioprocess outputs, - inputting in the model the real-time information on the bioprocess including bioprocess inputs and bioprocess outputs, - generating model outputs responsive to the real-time information on the bioprocess inputs using a dynamic and adaptive data-based on-line modelling technique, wherein at least one of the bioprocess inputs used as model input, is physical activity of the human or animal and at least one of the model outputs is an estimated component of the heart rate (HR mech), being a bioprocess output relating to the physical component of the heart rate, based on the physical activity, and - measuring the heart rate (HR tot) of the individual human or animal, for obtaining a measured heart rate (HR tot), - further comprising the step of using a value of an unknown variable to monitor and/or control the arousal component of the heart rate (HR PhAr), - y(t) is a time series of the real-time measured heart rate (HR tot); - t is time; - u(t-nk) is a time series of the bioprocess input, that is the physical activity of the human or animal, used as model input; - nk is a time delay between the bioprocess input and the bioprocess output; - e(t) is a dynamic error term describing the unknown variable attributed to the estimation of the arousal component of the heart rate (HR PhAr); - B(q), F(q), C(q) and D(q) are polynomials; - q is a backward shift operator.