Cold showers, cold plunges, and longer amounts of time outside in the cold. Look up Dr Jack Kruse or nostr:nprofile1qqswp5nf0tcpt6psn5e4g95ldg99lt3egty6j6urytkml9vcyld5amgprpmhxue69uhkummnw3ezumr0vdskc6r0wd6zuun9qyxhwumn8ghj7mn0wvhxcmmvuj5rph work. There a bunch of great podcast they’ve done