Susanna Söeberg's work keeps appearing in this knowledge base for good reason. She's one of the few researchers who has built a PhD specifically around cold water immersion and brown fat — not as a side topic, not as a footnote in a broader metabolic study, but as the central focus. And what she's arguing here isn't complicated: cold water doesn't just feel invigorating. It physically changes how your body generates and burns energy.
The mechanism is norepinephrine. When cold water hits your skin at 15 degrees Celsius or below, your sympathetic nervous system releases norepinephrine, which in turn activates brown adipose tissue — the fat that burns calories to produce heat rather than storing energy. This isn't the inert white fat we typically think of when we talk about body composition. Brown fat is metabolically active. It responds to cold. It responds to norepinephrine. And it responds to consistency.
The 2025 scoping review on cold water immersion sitting in the knowledge base confirms what Berg is describing — the physiological cascade from cold exposure includes enhanced circulation, improved immune markers, and genuine recovery benefits that hold up across multiple study designs. The mental health signal is also consistent. Reduced anxiety, improved mood, that particular afterglow of relaxation following acute cold stress. These aren't anecdote. They're reproducible findings.
What I find especially compelling from Berg's deeper work — visible in the Söeberg/The Proof transcript also in this knowledge base — is the interplay between the cold shock response and the diving reflex. When you submerge, particularly the face and neck, you activate a parasympathetic override. Heart rate drops. Calm follows. It's the body's evolutionary mechanism for surviving underwater, repurposed as a deliberate stress reset.
There's broad agreement on the metabolic and neurochemical effects — norepinephrine, brown fat activation, dopamine elevation. The 250% dopamine figure Berg cites is consistent with what Huberman's work also references. The disagreement tends to surface around protocol: how cold, how long, how often. Berg's guidance is notably gentle compared to more aggressive cold exposure advocates. She's not pushing ice baths at 2 degrees. She's working with 15 degrees Celsius, which is cold enough to trigger the response without becoming a test of pain tolerance.
That restraint is worth taking seriously. Hormesis — the principle that the right dose of stress builds resilience — only works if you recover from it. Too cold, too long, too often, and you're not adapting. You're depleting.
Start at 15 degrees Celsius for two to three minutes, three times a week. Focus on the breath before you enter — slow exhales to calm the sympathetic response before it spikes. Exit before you start shivering uncontrollably. Warm up naturally, or with movement, rather than immediately jumping into a hot shower, which truncates the brown fat activation window.
Berg's point about partial immersion is underappreciated: hands in cold water still triggers the response. If full immersion feels like too much, the hands and forearms are a legitimate starting point. The body doesn't require heroism to adapt. It requires repetition.
Here's what strikes me most: the 250% dopamine increase from cold immersion isn't just a mood stat. Dopamine is your motivation architecture. It drives the anticipation of reward, the willingness to do hard things, the baseline drive that separates people who follow through from people who don't. A consistent cold practice doesn't just make you feel better after the plunge. It recalibrates how you approach difficulty in general. That's not woo-woo — that's neurochemistry. And it's one of the more underappreciated arguments for making cold exposure a regular ritual rather than an occasional challenge.