Interactive systems are increasingly being used to explicitly support change in the user’s psychophysiological state and behavior. One trend in this vein is systems that support calm breathing habits. We designed and evaluated techniques to support respiratory regulation to reduce stress and increase parasympathetic tone. Our study revealed that auditory guidance was more effective than visual at creating self-reported calm. We attribute this to the users’ ability to effectively map sound to respiration, thereby reducing cognitive load and mental exertion. Interestingly, we found that visual guidance led to more respiratory change but less subjective calm. Thus, motivating users to exert physical or mental efforts may counter the calming effects of slow breathing. Designers of calming technologies must acknowledge the discrepancy between mechanical slow breathing and experiential calm in designing future systems.