Dr. Susanna Soberg is making a deceptively simple claim: that sitting in heat or cold is a form of exercise. Not a metaphor for exercise. Not a supplement to exercise. A cardiovascular stimulus that your heart, vessels, and metabolism cannot distinguish from mild aerobic work. Fifteen minutes in the sauna and your heart rate climbs, plasma volume expands, blood flow redirects. Your body is working, even though you're doing nothing but sitting.
But the deeper claim is about brown fat. And that's where this conversation gets interesting.
The FoundMyFitness body of work — Rhonda Patrick's deep dives into sauna and mitochondrial biogenesis — approaches this from the heat side and reaches similar conclusions about metabolic adaptation. But Soberg's Norwegian winter swimmer research adds the cold side of the equation in a way that's rarely been studied prospectively. These weren't people doing brief dips for Instagram. They were regular cold water swimmers, and their insulin sensitivity and glucose clearance markers were measurably better than controls.
The contrast therapy literature extends this further. When you alternate between heat and cold — the classic Nordic protocol — you're not just stacking two separate benefits. You're creating a vascular pump. The dilation from heat followed by the constriction from cold trains your vasculature to be more responsive, more elastic. The Finnish cardiovascular mortality data and Soberg's metabolic findings are likely telling the same underlying story: thermal stress builds resilience in the circulatory system.
There's broad consensus on the cardiovascular and metabolic benefits of sauna — the Finnish cohort studies are robust, replicated, and frankly hard to argue with. The cold exposure research is more contested. Duration matters enormously, and Soberg is careful here: short exposure activates brown fat, but prolonged exposure adds no benefit and may introduce risk. This is a point where the "more is better" crowd often goes wrong.
The one genuine area of ongoing debate is timing — specifically, whether cold exposure immediately after strength training blunts hypertrophy by suppressing the inflammatory signaling that drives muscle adaptation. Soberg doesn't address this directly, but it's worth knowing if your goals extend beyond metabolic health.
Three times a week, minimum. Fifteen to twenty minutes of heat, followed by two to three minutes of cold. End on cold if metabolic activation is your goal — Soberg's research suggests this is when brown fat does its most useful work. Don't obsess over water temperature. Cold enough to feel genuinely uncomfortable is cold enough. Consistency over heroics.
Soberg mentions sleeping in a cooler room as a brown fat activator, almost in passing. But think about what that implies. You don't need a plunge pool or a sauna to begin shifting your metabolic baseline. You need to stop overheating your bedroom at night. The threshold for brown fat activation is lower than most people assume — around 19 degrees Celsius. That's a blanket-off decision, not a cold plunge commitment. If you're not ready for deliberate cold exposure, start there. Your metabolism is already responding to ambient temperature, whether you're intentional about it or not.