Varför alla hoppar i kallt vatten efter bastun

Scroll through wellness content on any social media platform and you will inevitably come across it: someone stepping out of a sauna, steam rising from their skin, and dropping into a barrel of ice-cold water. The expression on their face — a mixture of shock, determination, and something close to joy — has become the defining image of the modern wellness movement.

But what is actually happening in the body during this ritual? And why are so many people — from elite athletes to office workers seeking stress relief — doing it?

The Ancient Practice With Modern Science

The combination of heat and cold is not a new idea. Nordic peoples have been jumping from saunas into frozen lakes for centuries. The Japanese ofuro bath tradition involves similar hot-cool contrasts. Ottoman hammams were designed around the contrast between hot rooms and cool marble slabs. The Roman thermae had caldarium (hot room) and frigidarium (cold room) as their core architecture.

What is new is the level of scientific attention this practice has received — and the precision with which researchers can now describe what happens inside the body.

The Science of the Plunge

When you step out of a sauna, your core temperature is elevated, your blood vessels are dilated, and your heart is beating faster than at rest. Every capillary near your skin surface is open, flooding your peripheral tissues with warm blood.

When you enter cold water — ideally below 15°C, and ideally quite suddenly — several things happen simultaneously:

Vasoconstriction. Your blood vessels contract rapidly, pushing blood from your extremities into your core. This protects your vital organs and helps regulate core temperature.

Norepinephrine spike. Cold exposure triggers a massive release of norepinephrine — a neurotransmitter and hormone that plays a key role in focus, attention, mood, and metabolism. Research by Dr. Susanna Søberg and others has shown that brief cold immersion can increase norepinephrine levels by 200–300%.

Dopamine elevation. Cold plunging also produces a sustained rise in dopamine — the neurotransmitter associated with motivation, reward, and wellbeing. Unlike the dopamine hit from eating sugar or checking social media (which spikes fast and drops fast), the dopamine elevation from cold exposure lasts for hours.

Endorphin release. The shock of cold triggers beta-endorphin release — the same chemicals produced during aerobic exercise and responsible for the "runner's high."

The Søberg Protocol

Dr. Susanna Søberg's research at the University of Copenhagen produced one of the most practically useful findings in cold therapy science. Her 2021 study found that a minimum of 11 minutes of cold water immersion per week — spread over multiple sessions — was sufficient to produce significant metabolic benefits, including activation of brown adipose tissue (brown fat).