Heart rate change has been used in a few previous studies to evaluate acute stress response, although primarily in indoor laboratory settings. (sciencedaily.com)
Associations between DNA methylation of a glucocorticiod receptor promoter and acute stress responses in a large healthy adult population are largely explained by lifestyle and educational differences (cambridge.org)
This is known as the relaxation response, and it directly combats that acute stress response mentioned earlier, helping the body achieve balance. (mindbodygreen.com)