The plunge works because it is uncomfortable and short. What ruins it is lukewarm water, oversized chillers, and macho protocols. We size the setup and keep the routine honest.
7°C · held steady
A chiller that is too small never reaches temperature in summer. Too big wastes money you could spend on the tub. Size it from your water volume and your climate, not from a bundle page.
Cold enough to gasp, warm enough to stay in control. The badge-of-honor temperatures can wait a month.
The gasp reflex is real. A long exhale as you step in keeps breathing in your hands, not your brainstem's.
Most of the measurable response happens early. Three short plunges beat one shivering marathon.
Let the shiver do its work, then walk or do light squats. Save the sauna for later so the cold signal lands.