Cold plunging is deliberate exposure to cold water (typically 38-55°F) for short durations (2-5 minutes).
Mechanism: Activates the sympathetic response, releases norepinephrine and dopamine (sustained for hours), promotes mitochondrial biogenesis, may induce brown adipose tissue. Best evidence is for mood and recovery; lipid and longevity benefits are more speculative.
Use case: 11 minutes per week, split across sessions, 38-55°F. Best in the morning. Don't do it within 4 hours after hypertrophy training. Cold blunts muscle protein synthesis.
Caveats: Don't cold-plunge with untreated cardiovascular conditions. Start with 1-minute exposures and build. Drowning risk is real if you go too cold too fast.