Six competitive runners sat in a sauna for 30 minutes after training, three times a week, for three weeks. Their time to exhaustion improved by 32%.
Not 3.2%. Thirty-two percent.
That was 2007. A small crossover study from New Zealand, published in the Journal of Science and Medicine in Sport, that most of the endurance world ignored. The researchers found that plasma volume increased by 7.1% and that the correlation between plasma volume change and performance change was 0.96 — nearly perfect.
The finding sat quietly in the literature for over a decade. Then professional cycling noticed.
In 2020, Bent Rønnestad's lab at Inland Norway University of Applied Sciences ran a controlled trial with 23 elite male cyclists — athletes with a mean VO₂max of 76.2 mL·min⁻¹·kg⁻¹, which puts them firmly at international level. Five weekly sessions of 50 minutes in a heat chamber at ~38°C for five weeks produced a measurable increase in haemoglobin mass, with small to intermediate effect sizes favouring heat training across lactate threshold power output, gross economy in a fatigued state, and 15-minute mean power. The study was published in Experimental Physiology.
Why does haemoglobin mass matter? Because it is one of the strongest determinants of VO₂max. More haemoglobin means more oxygen-carrying capacity per unit of blood. Altitude camps exist for precisely this reason — and they cost professional teams upwards of £50,000 per block.
Rønnestad's follow-up in 2022, published in Medicine & Science in Sports & Exercise, showed that the haemoglobin mass gains (~2.5%) could be maintained by reducing to just three heat sessions per week for three weeks. A further 2025 study demonstrated that three weekly heat sessions after an altitude camp preserved the elevated haemoglobin mass that would otherwise decay within weeks of returning to sea level.
This is no longer a fringe finding. WorldTour teams including Red Bull–BORA–hansgrohe are integrating heat training into their periodisation. The protocol is accessible to anyone with a gym sauna.
What's actually happening physiologically
Three mechanisms drive the performance gains. Each operates on a different timescale, and together they explain why post-exercise heat exposure is more than a recovery ritual.
Plasma volume expansion — the fast response. Within the first week of post-exercise sauna use, plasma volume begins to increase. A 2015 study tracking seven well-trained cyclists over 35 consecutive days found peak plasma volume expansion after just four sauna exposures. More plasma means higher stroke volume, which means more blood pumped per heartbeat. The downstream effect: lower heart rate at submaximal intensities and a larger cardiac reserve when you need it.
Every cyclist knows what this feels like. The day your heart rate sits 5 beats lower at threshold power is the day something has shifted. The sauna accelerates that shift without adding a single kilojoule of training stress.
Haemoglobin mass — the altitude-camp effect. Heat stress triggers erythropoietin (EPO) production — the same hormone your body releases at altitude. The Rønnestad data in elite cyclists showed ~2.5% increases in haemoglobin mass over five weeks. That may sound modest, but in athletes already operating at the physiological ceiling, a 2.5% gain in oxygen-carrying capacity is the difference between making the selection and watching from home.
The practical significance for age-group cyclists and serious amateurs: you are further from your physiological ceiling, which means the relative gains may be even larger. The Kirby study in trained middle-distance runners — athletes closer to a typical serious amateur level — showed an ~8% improvement in VO₂max and ~12% improvement in time to exhaustion over three weeks. Those numbers are not marginal. They are transformational.
Heat shock proteins — the cellular repair crew. When core temperature rises, cells upregulate a family of proteins called heat shock proteins, particularly HSP70. These proteins act as molecular chaperones — they repair misfolded proteins, protect cells from oxidative damage, and activate the mTOR signalling cascade that drives muscle protein synthesis. The anti-catabolic effect is equally important: HSP70 inhibits the ubiquitin-proteasome pathway, slowing the breakdown of damaged muscle proteins after hard sessions.
For the cyclist managing 10 hours of training around a demanding job, this is the mechanism that matters most. You are not adding training load. You are increasing the return on training load you have already invested.
The cognitive dividend most athletes miss
Here is where the data gets interesting for anyone who makes decisions for a living.
The landmark Finnish Kuopio Ischemic Heart Disease study — a 20-year prospective cohort of 2,315 middle-aged men published in JAMA Internal Medicine — found that men who used a sauna 4–7 times per week had a 40% lower all-cause mortality compared to those who used it once a week. A subsequent analysis from the same cohort, published in Age and Ageing, found a 66% reduction in dementia risk for the most frequent sauna users.
The mechanistic explanation is increasingly clear. Heat exposure elevates brain-derived neurotrophic factor (BDNF) — a protein critical for neuroplasticity, memory formation, and cognitive maintenance. It also drives a roughly threefold increase in norepinephrine, the neurotransmitter most directly responsible for focus and sustained attention. A 2018 study in the International Journal of Hyperthermia showed that hot water immersion alone was sufficient to increase serum BDNF levels in healthy males [SOURCE: VERIFY — Kojima et al., International Journal of Hyperthermia, 2018, 34(6), 834–839].
Put this together with the post-exercise context and the implication is striking. The 20 minutes you spend in a sauna after a hard ride are simultaneously building your aerobic engine and sharpening the cognitive machinery you rely on in every board meeting, negotiation, and high-stakes decision.
Your power meter does not care about your job title. But your prefrontal cortex does care about blood flow, BDNF, and norepinephrine. The sauna delivers all three.
The Post-Ride Heat Protocol
Based on the current evidence, here is a framework you can implement this week:
Temperature: 80–100°C (traditional Finnish sauna). The Kirby study used 101–108°C; the Scoon study used ~90°C. If your gym sauna runs cooler, extend duration slightly.
Duration: 15–30 minutes. The strongest evidence clusters around 25–30 minutes. If you are new to post-exercise sauna, start with 15 minutes and build over two weeks.
Timing: Immediately post-exercise. The studies showing the largest effects all used sauna exposure directly after training, not hours later. The physiological window matters.
Frequency: Three sessions per week minimum. The Kirby data showed meaningful adaptation in three weeks at this frequency. The Rønnestad maintenance data suggests three weekly sessions are sufficient to hold haemoglobin mass gains.
Hydration: Non-negotiable. You are already dehydrated from training. Aggressive fluid replacement before, during (if your sauna permits), and after. Electrolytes, not just water.
Duration of programme: Minimum three weeks for measurable physiological adaptation. The evidence suggests most gains plateau by five to seven weeks, so this is not a permanent daily commitment — it is a targeted block you periodise like any other training stimulus.
The honest caveats
The most comprehensive systematic review to date — published in Frontiers in Sports and Active Living in 2025 — rated the overall quality of evidence as low to moderate, and noted that repeated heat exposures had no statistically significant effect on cycling-specific VO₂max. The strongest performance evidence is in running. The haemoglobin mass data in elite cyclists is robust, but the direct transfer to time-trial performance is less clear-cut.
This is not a magic protocol. It is a marginal gain with a genuine physiological basis and a remarkably low barrier to entry.
The question is not whether 15 minutes in a sauna will replace structured training. It will not. The question is whether you are already doing the hard sessions, already managing the load, already optimising the inputs — and leaving the cheapest, lowest-risk adaptation trigger on the table.
For most of us, the answer is yes.
Flowstate AI is now live.
The hard part of applying this framework is not the physical discipline. The hard part is the integration — knowing when to push, when to protect the empty vessel, and how to read the signals before your performance degrades.
Flowstate AI connects your Garmin data, training load, and performance patterns to give you a real-time picture of your cognitive and physical readiness.
No one can ride your intervals for you. But the data helps you ride the right ones.
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