There is a quiet revolution happening in metabolic science, and it begins with a simple question: how little cold and heat exposure do you actually need to change your biology?
Dr. Susanna Søberg, the researcher behind the now-famous Søberg Principle, has spent years studying the intersection of cold water immersion, sauna use, and human metabolism. In a conversation with Jesús Sierra, she distilled her PhD research into remarkably accessible protocols — protocols that require far less time than most people assume.
Her findings suggest that just 11 minutes of cold water and 57 minutes of sauna per week, distributed across two to three sessions, can meaningfully improve insulin sensitivity, glucose clearance, and brown fat activation. The key is not duration. It is consistency, contrast, and one counterintuitive principle: always end on cold.
When the body encounters cold water, its first response is sympathetic activation — heart rate climbs, metabolic rate accelerates. But the deeper mechanism is what Dr. Søberg finds most compelling: the activation of brown adipose tissue.
Unlike white fat, which stores energy, brown fat burns it. Brown fat cells pull glucose and fatty acids directly from the bloodstream to generate heat. This process continues even after you leave the water, as the body adapts and prepares for future cold exposures.
Dr. Søberg draws a direct parallel to exercise: "It is just like when you go to the gym. You use your muscles, they break down a little, and then they rebuild stronger. The same hormetic stress happens when you go into cold water or the sauna."
Beyond metabolism, cold exposure delivers something people feel immediately: a shift in mood. Dopamine and noradrenaline surge within seconds of cold contact, creating what Dr. Søberg calls the "Control-Delete effect" — a complete neurochemical reset.
This is not subtle. A 30-second cold shower at the end of your morning routine can provide the drive and clarity to navigate the entire day. The energy is not nervous or jittery. It is the kind of sustained alertness that makes you more present, more approachable, more engaged.
The timing matters, though. Cold exposure in the morning harnesses these neurotransmitters when they serve you best. In the evening, that same dopamine and noradrenaline spike would disrupt sleep — just like coffee after 5 PM.
On the heat side, the evidence is equally striking. The Finnish Sauna Cohort followed 2,300 regular sauna users for over twenty years, measuring cardiovascular outcomes against frequency of use.
The results showed a clear dose-response relationship. Those who used sauna two to three times per week had a 39% lower risk of cardiovascular disease compared to once-weekly users. Those who used sauna four to seven times per week saw their risk of early death drop by nearly 50%.
What makes this data particularly noteworthy is the control group: these were not sedentary individuals being compared to sauna users. The baseline was people who already used sauna once a week. The additional sessions produced additional protection — a cardiovascular workout that requires no movement at all.
Dr. Søberg's PhD research focused specifically on contrast therapy — the deliberate alternation between cold and heat. Her findings suggest that the combination is more powerful than either modality alone.
The protocol is straightforward: begin with cold, alternate with heat, and always finish on cold. This forces the body to reheat itself naturally, which demands additional energy expenditure — essentially extending the metabolic benefit long after the session ends.
Her adapted winter swimmers showed better insulin sensitivity and faster glucose clearance than controls who exercised but did not practice thermal therapy. The weekly minimums were remarkably modest: 11 total minutes of cold water immersion and 57 total minutes of sauna, each divided across two to three days with multiple short exposures per session.