An electronic processing system designed to real-time detect one or more of awake (W), Drowsiness (D), and Sleep (S) phases of a subject, and/or predict transitions between at least awake (W) phase and either Drowsiness (D) or Sleep (S) phase of the subject based on photoplethysmography (PPG) technology and related physiological parameters of the subject measured via either a contact or a contactless photoplethysmography (PPG) sensor ; the electronic processing system is programmed to: acquire a PPG signal from either a contact or a contactless PPG sensor ; analyse the PPG signal waveform in one or both of time and frequency domains; and performing at least one of the following: detecting one or more of awake (W), Drowsiness (D), and Sleep (S) phases of a subject based on an output of the analysis, and predicting transitions between at least awake (W) phase and either Drowsiness (D) or Sleep (S) phase of the subject based on an output of the analysis. characterised in that the electronic processing system is further programmed to analyse the PPG signal waveform in the frequency domain by: computing a Power Spectrum Density (PSD) of the PPG signal waveform, computing a maximum amplitude of the PSD in a lower frequency range thereof and a maximum amplitude of the PSD in a higher frequency range thereof, and computing a ratio ( λ ) between the computed maximum amplitudes; and in that the electronic processing system is further programmed to predict onset of sleep in the subject based on a behaviour of the computed ratio ( λ ) over time. acquire a PPG signal from either a contact or a contactless PPG sensor ; analyse the PPG signal waveform in one or both of time and frequency domains; and performing at least one of the following: detecting one or more of awake (W), Drowsiness (D), and Sleep (S) phases of a subject based on an output of the analysis, and predicting transitions between at least awake (W) phase and either Drowsiness (D) or Sleep (S) phase of the subject based on an output of the analysis. detecting one or more of awake (W), Drowsiness (D), and Sleep (S) phases of a subject based on an output of the analysis, and predicting transitions between at least awake (W) phase and either Drowsiness (D) or Sleep (S) phase of the subject based on an output of the analysis. characterised in that the electronic processing system is further programmed to analyse the PPG signal waveform in the frequency domain by: computing a Power Spectrum Density (PSD) of the PPG signal waveform, computing a maximum amplitude of the PSD in a lower frequency range thereof and a maximum amplitude of the PSD in a higher frequency range thereof, and computing a ratio ( λ ) between the computed maximum amplitudes; and in that the electronic processing system is further programmed to predict onset of sleep in the subject based on a behaviour of the computed ratio ( λ ) over time. computing a Power Spectrum Density (PSD) of the PPG signal waveform, computing a maximum amplitude of the PSD in a lower frequency range thereof and a maximum amplitude of the PSD in a higher frequency range thereof, and computing a ratio ( λ ) between the computed maximum amplitudes; and in that the electronic processing system is further programmed to predict onset of sleep in the subject based on a behaviour of the computed ratio ( λ ) over time. <EOS>