Interactive immersive Virtual Reality environments have been shown to be beneficial in healthcare and mental wellbeing by enabling effective interventions such as stress management and behavioural therapy. Our goal is to build an automated breathing relaxation system that is based on a closed-loop, implicit interaction and that harnesses known mechanisms of physiological synchrony found in human-human interaction. In the Virtual Human Breathing Relaxation System the user's breathing rhythm is influenced using an ecologically valid physiological signal, i.e. the breathing behaviour of a virtual human. By continuously adapting the virtual human's breathing rhythm the system aims to bring the participants breathing behaviour to a rhythm that allows them to feel more relaxed.