This document discloses a solution for estimating a stress level of a human by using heart activity measurement data. According to an aspect, a method comprises: method for estimating a stress level of a human by an apparatus, comprising: 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.
