A method (1500) for deriving a mental state of a subject is disclosed. The method comprises receiving (1502) a bio-signal from the subject calculating respective statistical variations of at least two physiological parameters derived from the bio-signal determining (1504) an arousal level of the subject based on the calculated statistical variations of the at least two physiological parameters deriving a time-domain heart rate variability signal from the bio-signal for calculating at least two heart rate variability parameters determining (1506) a valence level of the subject based on the at least two heart rate variability parameters and deriving (1508) the mental state from the arousal level and valence level. A related apparatus is also disclosed.
