There are a number of articles written based on research conducted by Redwine et al. (2016) which links gratitude to HRV. See Allen (2018) for an example.
EliteHRV (2018) stated that “This line of research began in 1995, when a study found that people feeling appreciation (an emotion related to gratitude) have improved heart rate variability”
As Redwine et al. (2016) state more research needs to be done, however, there is enough anecdotal evidence to suggest that practising gratitude can have a positive effect for some.
Allen, S 2018, Is Gratitude Good for Your Health?, viewed 3 December 2018, <https://greatergood.berkeley.edu/article/item/is_gratitude_good_for_your_health>.
EliteHRV 2018, HRVcourse onTwitter, viewed 3 Dec 2018, <https://twitter.com/hrvcourse/status/1068150364116004864>.
Redwine, LS, Henry,BL, Pung, MA, Wilson, K, Chinh, K, Knight, B, Jain, S, Rutledge, T, Greenberg,B, Maisel, A & Mills, PJ 2016, ‘Pilot Randomized Study of a GratitudeJournaling Intervention on Heart Rate Variability and Inflammatory Biomarkersin Patients With Stage B Heart Failure’, PsychosomMed, vol. 78, no. 6, 2016 Jul-Aug, pp. 667-76.