A portable device for the monitoring of sleep of a user, comprising: a photoplethysmographic (PPG) sensor intended to be in contact with a tissue of the user and adapted for providing a PPG signal; a motion detecting device for measuring motion of the device on and with respect to the user body tissue and for providing a motion reference signal representative of said motion and determining a user activity from the motion reference signal; and a processing device configured for determining beat-to-beat intervals from the PPG signal; the processing device being further configured for determining heart rate variability using the determined beat-to-beat intervals and determining a sleep-related parameter from the determined heart rate variability and the user activity, and for using the motion reference signal in combination with said beat-to-beat intervals to determine a sleep-related parameter of the user, wherein the sleep-related parameter comprises, activity of the autonomic nervous system of the user while sleeping, or a sleep stage of the user; characterized in that the processing device is further configured for using the motion reference signal to select reliable portions of the PPG signal when the motion reference signal is below a threshold movement value (Th) and to reject portions of the PPG when the motion reference signal is equal or above the threshold movement value (Th); and in that the processing device is further configured for determining beat-to-beat intervals from a time-series of beat-to-beat intervals and determining the position of the heart beats by searching minimal value of the time-derivative of the selected reliable portions of the PPG signal. a photoplethysmographic (PPG) sensor intended to be in contact with a tissue of the user and adapted for providing a PPG signal; a motion detecting device for measuring motion of the device on and with respect to the user body tissue and for providing a motion reference signal representative of said motion and determining a user activity from the motion reference signal; and a processing device configured for determining beat-to-beat intervals from the PPG signal; the processing device being further configured for determining heart rate variability using the determined beat-to-beat intervals and determining a sleep-related parameter from the determined heart rate variability and the user activity, and for using the motion reference signal in combination with said beat-to-beat intervals to determine a sleep-related parameter of the user, wherein the sleep-related parameter comprises, activity of the autonomic nervous system of the user while sleeping, or a sleep stage of the user; characterized in that the processing device is further configured for using the motion reference signal to select reliable portions of the PPG signal when the motion reference signal is below a threshold movement value (Th) and to reject portions of the PPG when the motion reference signal is equal or above the threshold movement value (Th); and in that the processing device is further configured for determining beat-to-beat intervals from a time-series of beat-to-beat intervals and determining the position of the heart beats by searching minimal value of the time-derivative of the selected reliable portions of the PPG signal. <EOS>