Method and apparatus for deriving a mental state of a subject

A method for determining an arousal level of a subject, the method comprising:

• (i) receiving a bio-signal from a wearable device worn by the subject, the bio-signal including at least one selected physiological parameter, the at least one selected physiological parameter including pulse pressure;
• (ii) segmenting the bio-signal into a plurality of processing windows, each processing window having a time duration that is adaptively configured depending on the at least one selected physiological parameter to include at least five consecutive cardiac cycles of the bio-signal;
• (iii) calculating statistical variation of the at least one selected physiological parameter including pulse pressure derived from the at least five consecutive cardiac signals of the bio-signal;
• (iv) determining the arousal level of the subject based on the calculated statistical variation of the at least one selected physiological parameter including pulse pressure, wherein the arousal level is determined to be in a stress level if the calculated statistical variation of the pulse pressure is greater than a threshold value, in which the threshold value is within a range of about 3.0-12.0 mmHg mercury level, and
• (v) outputting the arousal level and, based on the determined arousal level, a training or an intervention to assist with managing the stress level.