Published Paper Details:

Abstract Background: Cold water immersion (CWI) and intermittent fasting (IF) are increasingly adopted as hormetic strategies to improve stress resilience, autonomic regulation, and sleep quality. Methods: A systematic review and meta-analysis of controlled trials in healthy adults (>=18 years) synthesized data from 11 CWI and 23 IF studies assessing stress markers, heart rate variability (HRV), and sleep outcomes. Standardized mean differences with 95% confidence intervals were calculated. Results: CWI produced a large time-dependent reduction in stress at 12 hours post-exposure, alongside significant improvements in vagally mediated HRV indices and slow-wave sleep with fewer nocturnal arousals. IF, particularly 16:8 time-restricted eating, improved HRV over 8-12 weeks and induced adaptive endocrine changes, including increased cortisol and reduced triiodothyronine. Direct evidence for combined CWI+IF protocols was limited to a single pilot study. Conclusions: CWI and IF independently benefit stress reactivity, autonomic nervous system function, and sleep via hormetic mechanisms, with preliminary theoretical support for synergistic effects when combined. High-quality trials on combined protocols, diverse populations, and long-term safety are warranted.