The vagus nerve has become an increasingly important focus in physiology and neuroscience because of its role in regulating heart rate, blood pressure, breathing, and communication between the brain and internal organs. One of the most reliable, non-invasive markers of this activity is Respiratory Sinus Arrhythmia (RSA), the natural variation in heart rate synchronised with the breathing cycle, which serves as a direct index of cardiac vagal control (Van den Berg et al., 2015). As interest in stress, recovery, and mind-body regulation continues to grow, so has attention to the possibility that vagus nerve activity may help explain important differences in mental and physical health. Over the past several years, vagus nerve stimulation has grown well beyond its older reputation as a niche implant-based treatment, with applications now spanning epilepsy, treatment-resistant depression, obesity, headache, and post-stroke rehabilitation. The same review also shows how non-invasive stimulation approaches have made the field easier to study in broader research and clinical settings. That expansion is visible in the literature itself: a 2024 bibliometric analysis found growing research activity around depression, pain, migraine, cognition, and markers of vagal tone like heart rate variability and RSA. What makes the area especially interesting now is that the science is becoming more precise. Researchers are moving beyond asking whether stimulation works at all and are increasingly studying who may benefit most and which physiological signals can help predict and quantify response. Because RSA functions as a sensitive metric for mapping stimulation outcomes, it is becoming essential for measuring direct changes in autonomic state. One recent study found that baseline heart rate variability and related vagal markers may help predict differences in response to transcutaneous auricular vagus nerve stimulation in depression research. Other reviews have also noted that vagus nerve stimulation has already entered clinical use in some stroke rehabilitation settings, highlighting how quickly the field is moving from exploratory research into practical application. Much more work is still needed before this momentum translates into a reliable everyday research practice. Recent work has shown that respiration rate, tidal volume, and speech can complicate the interpretation of general heart rate variability as a measure of cardiac vagal activity, making respiration-linked metrics like RSA critical to avoid skewed data. Related research on ambulatory psychophysiological measurement underlines why signal quality and study design matter. This is where VU-AMS is uniquely relevant: by providing an established, integrated physiological monitoring device, we support high-quality, non-invasive data acquisition. By delivering RSA as a core metric, we empower researchers to isolate true cardiac vagal activity from confounding factors and study autonomic changes with greater confidence in all settings. As vagus nerve research continues to expand, better outcomes will depend not only on new interventions but also on accurate methods for measuring the physiology underlying them.