Abstract
Meditation has become prominent in both clinical and non-clinical applications for its effects on psychological and physical well-being. Long-term meditators, who have dedicated extensive time to their practice, present a unique opportunity to explore the effects of prolonged meditation training on the autonomic nervous system. Research has reported concomitant activation of both sympathetic (aroused) and parasympathetic (relaxed) branches of the autonomic nervous system during some forms of meditation, leading to the term 'relaxed alertness.' However, findings are not consistent, with reports of both sympathetic and parasympathetic activation, sympathetic-only, parasympathetic-only, or temporally variable activations, depending on several factors. This review synthesizes these heterogeneous and seemingly inconsistent results in relation to three explanatory factors: (1) specific classification of style or type of meditation; (2) specific definition of the level of expertise of the meditators; and (3) intra-individual variations within a given meditation practice. When these factors are considered, convergent and meaningful patterns emerge, allowing for a shift from the broad notion of 'long-term' meditation to a more precise characterization of 'advanced' meditation, highlighting skills, states, and stages of mastery developed over time. Our synthesis is particularly useful for understanding both long-term and advanced meditation, as it reveals specific heart rate variability patterns, including very low and low-frequency spectral power peaks, along with cardiac and respiratory coupling. Better characterization of the role of the autonomic nervous system in the context of advanced meditation promises to inform improved meditation training, including training assisted by technology, toward more impactful outcomes.