Altered states of consciousness (ASCs), induced e.g. by psychedelics, show great potential to treat widespread mental health disorders like depression and PTSD. However, access to such treatments is restricted by legal, medical, and financial barriers. Here, we explore the potential of circular breathwork to serve as an accessible, non-pharmacological alternative to engage similar therapeutic processes. Scientific studies investigating the mental health effects of breathwork are only just emerging and the underlying physiological and psychological mechanisms are largely unknown. In this study, we address these questions by tracking physiological and experiential dynamics throughout a breathwork session, comparing two popular forms of breathwork: Holotropic Breathwork® and Consciously-Connected breathwork. We show that a reduction in end-tidal CO2 pressure due to deliberate hyperventilation is instrumental in catalyzing ASCs during breathwork. ASCs evoked by breathwork were comparable to those produced by psychedelics, and their depth predicted psychological and physiological follow-on effects, including improved well-being and a reduction of depressive symptoms. Moreover, different breathwork formats produced highly similar physiological, experiential and psychological outcomes. These results outline physiological boundary conditions for ASCs to arise in a non-pharmacological context, shedding light on the functional mechanisms of breathwork as well as its potential as a psychotherapeutic tool.