Abstract: A method of estimating a stress level of a human by an apparatus using heart activity measurement data includes: executing a breathing exercise application by the apparatus, starting a breathing exercise of the breathing exercise application and outputting, during the breathing exercise, breathing instructions to a user of the apparatus; acquiring a set of heart activity measurement data samples measured by a heart activity sensor from the user during the breathing exercise; computing a set of inter-heartbeat interval samples of the set of heart activity measurement data samples; computing a cardiac coherence of the user during the exercise from the set of inter-heartbeat interval samples; measuring a respiratory rate of the user during the breathing exercise; computing a score of the breathing exercise on the basis of the respiratory rate and the cardiac coherence, the score indicating a stress level of the user; and outputting the score through an interface of the apparatus.