Human physiological responses to immersion into water of different temperatures. - PubMed - NCBI
"...Plasma noradrenaline and dopamine concentrations were increased by 530% and by 250% respectively"
Surprising Health Benefits of Extreme Hot and Cold Temperatures
"You can increase norepinephrine two-fold just by getting into 40-degree water for 20 seconds, or 57-degree water for a few minutes.
While best known as a neurotransmitter, norepinephrine also acts as a hormone. One of its functions is causing vasoconstriction, which helps your body conserve heat. Norepinephrine also acts as a signaling molecule to make more mitochondria in your fat tissue (your main energy reserves)"