The study below examines a wide-variety of breathwork methods.
Fincham, G. W., Strauss, C., Montero-Marin, J., & Cavanagh, K. (2023). Effect of breathwork on stress and mental health: A meta-analysis of randomised-controlled trials. Scientific Reports, 13(1), 432
A summary of the study with additional references was published in the Journal of Cognitive Psychology in 2024.
The study recognises statistically significant small-to-medium effects on stress, anxiety and depression.