The practice, honestly
Circulation
Heat dilates blood vessels; cold constricts them. Alternating the two is a simple, well-understood way to train vascular response. The research direction is genuine; the magnitude depends on the person.
Recovery
Cold water immersion after exercise is a standard practice in sports medicine for easing muscle soreness. Evidence of a big performance boost is mixed — we'd rather be honest than oversell.
Norepinephrine & mood
Human trials show cold water immersion is associated with a marked rise in norepinephrine and a reported mood lift. Numbers vary widely by study and protocol.
Sleep
Many practitioners report deeper sleep after a heat–cold cycle. The mechanism isn't settled, and everyone's rhythm differs.
Where to look
Where to look: PubMed-indexed reviews of cold water immersion and contrast therapy, and the sports-medicine literature on post-exercise recovery. Start there, and treat blog claims with the same skepticism you'd bring to any health advice.