A processor implemented method, comprising: obtaining by one or more hardware processors instantaneous HR from a resampled time series of RR intervals of an ECG signal specific to user, wherein the instantaneous HR is obtained for a plurality of time windows from the resampled time series; computing, by one or more hardware processors for each time window of the plurality of time windows, a measured average heart rate (a measured HR) from the instantaneous HR specific to the user; inputting, by one or more hardware processors to a Kalman Filter, the measured average heart rate (the measured HR) and an initial estimate of an ideal HR, wherein the Kalman Filter comprises a state space model that is designed based on baseline dynamics of an average heart rate; for each time window until a last time window of the plurality of time windows, performing: estimating by one or more hardware processors an apriori state based on a last instance of an aposteriori state being initialized, wherein the initialized aposteriori state is based on the initial estimate of the ideal HR fed to the Kalman filter; determining, by one or more hardware processors using the apriori state, an observation of the state space model of the Kalman filter; estimating by one or more hardware processors an error based on the observation and the measured average heart rate (the measured HR), and normalizing the error thereof; performing by one or more hardware processors a comparison of the normalized error with a pre-defined threshold; and continually monitoring by one or more hardware processors an arousal of the user based on the comparison to obtain an arousal trend. obtaining by one or more hardware processors instantaneous HR from a resampled time series of RR intervals of an ECG signal specific to user, wherein the instantaneous HR is obtained for a plurality of time windows from the resampled time series; computing, by one or more hardware processors for each time window of the plurality of time windows, a measured average heart rate (a measured HR) from the instantaneous HR specific to the user; inputting, by one or more hardware processors to a Kalman Filter, the measured average heart rate (the measured HR) and an initial estimate of an ideal HR, wherein the Kalman Filter comprises a state space model that is designed based on baseline dynamics of an average heart rate; for each time window until a last time window of the plurality of time windows, performing: estimating by one or more hardware processors an apriori state based on a last instance of an aposteriori state being initialized, wherein the initialized aposteriori state is based on the initial estimate of the ideal HR fed to the Kalman filter; determining, by one or more hardware processors using the apriori state, an observation of the state space model of the Kalman filter; estimating by one or more hardware processors an error based on the observation and the measured average heart rate (the measured HR), and normalizing the error thereof; performing by one or more hardware processors a comparison of the normalized error with a pre-defined threshold; and continually monitoring by one or more hardware processors an arousal of the user based on the comparison to obtain an arousal trend. estimating by one or more hardware processors an apriori state based on a last instance of an aposteriori state being initialized, wherein the initialized aposteriori state is based on the initial estimate of the ideal HR fed to the Kalman filter; determining, by one or more hardware processors using the apriori state, an observation of the state space model of the Kalman filter; estimating by one or more hardware processors an error based on the observation and the measured average heart rate (the measured HR), and normalizing the error thereof; performing by one or more hardware processors a comparison of the normalized error with a pre-defined threshold; and continually monitoring by one or more hardware processors an arousal of the user based on the comparison to obtain an arousal trend.