Variability in heart rate at rest that is associated with respiratory patterns. Vagal tone reflect the functioning of the vagus nerve, which is the 10th
cranial nerve and a core component of the parasympathetic nervous system, which regulates heart rate in response to signals of safety and interest.
Kok, B. E., Coffey, K. A., Cohn, M. A., Catalino, L. I., Vacharkulksemsuk, T., Algoe, S. B., ... & Fredrickson, B. L. (2013). How positive emotions build physical health perceived positive social connections account for the upward spiral between positive emotions and vagal tone. Psychological Science, 24(7), 1123-1132. doi: 10.1177/0956797612470827