Context. Many reports support the fact that some psycho--social aspects of software engineers are key factors for the quality of the software development process and its resulting products. Based on the experience of some of the authors after more than a year of practising mindfulness---a meditation technique aimed to increase clearness of mind and awareness---we guessed that it could be interesting to empirically evaluate whether mindfulness affects positively not only the behaviour but also the professional performance of software engineers. Goal. In this paper, we present a quasi--experiment carried out at the University of Seville to evaluate whether Software Engineering & Information Systems students enhance their conceptual modelling skills after the continued daily practice of mindfulness during four weeks. Method. Students were divided into two groups: one group practised mindfulness, and the other---the control group---were trained in public speaking. In order to study the possible cause--and--effect relationship, effectiveness (the rate of model elements correctly identified) and efficiency (the number of model elements correctly identified per unit of time) of the students developing conceptual modelling exercises were measured before and after taking the mindfulness and public speaking sessions. Results. The experiment results have revealed that the students who practised mindfulness have become more efficient in developing conceptual models than those who attended the public speaking sessions. With respect to effectiveness, some enhancement have been observed, although not as significant as in the case of efficiency. Conclusions. This rising trend in effectiveness suggests that the number of sessions could have been insufficient and that a longer period of sessions could have also enhanced effectiveness significantly.