HRV, or Heart Rate Variability, is the variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval. Through the HRV, you can measure stress in a simple, fast and reliable way.

The articles below are a selection of scientific articles that extensively discuss the fact that HRV is the most reliable bio marker to measure stress in the body.

Effects of heart rate variability biofeedback training in athletes exposed to stress of university examinations
July 26, 2018  
Veronique Deschodt-Arsac, Romain Lalanne, Beatrice Spiluttini, Claire Bertin, Laurent M. Arsac
Non-Linear Methods in HRV Analysis
October 2015
University of Medicine and Pharma of Tigru-Mures, 9thInternational Conference Interdisciplinarity in Engineering
Resonant Breathing Biofeedback Training for Stress Reduction Among Manufacturing Operators
January 2015
Auditya Purwandini Sutarto, Muhammad Nubli Abdul Wahab & Nora Mat Zin
Heart Rate Variability
From Wikipedia, the free encyclopedia
Historical description of HRV
2011
George E.Billmaa, Ohio State University, Columbus, OH, USA, University of Oulu, Finland: George
Proof of high degree of correlation between stress and HRV
2007
Information and Communication University, Inje University School of Medicine Seoul, Korea. Lizawati Salahuddin, Myeong Gi Jeong, Desok Kim, Seong-Kyeon Lim , Kim Won , Jong-Min Woo

 

Relationship between emotional burnout and HRV
December 2016
Lennartsson AK, Jonsdottir I, Sjörs
Comparative analysis of blood pressure and HRV as mental stress indicators
June 2004
National Institute for Occupational Safety and Health, Denmark: Hjortskov N1, Rissén D, Blangsted AK, Fallentin N, Lundberg U, Søgaard K.
Influence of Mental Stress on HR and HRV, HRV changes during mental stress
2008
University of Leuven, Belgium: J. Taelman, S. Vandeput, A. Spaepen, S. Van Huffel. Influence of Mental Stress on Heart Rate and Heart Rate Variability.
Heart rate variability: Standards of measurement, physiological interpretation, and clinical use 
Maart 1996
Marek Malik J. Thomas Bigger A. John Camm Robert E. Kleiger Alberto MallianiArthur J. Moss Peter J. Schwartz
European Heart Journal
HRV measurement standards
March 1996
Task Force of the European Society of Cardiology and the North American society of Pacing and Electrophysiology.