A beginner’s guide to sauna, sweating and the off-season
By Coach Joe Beer
Okay, so I am going to assume you are not already a guru of saunas for passive heat training, or even super-hot baths. I will guess you may already have done random active heat sessions, either as a deliberate “how hot can I get” session or getting caught out in hot weather races, now but a distant memory.
There is something nice about sitting in very warm conditions when the northern hemisphere turns from Autumn to Winter and temperatures drop. I remember using “as-long-as-I-can” sauna sessions in winter to just improve mood and in summer before racing in places like Muncie, Indiana and Nice, France. Plus again prior to racing in the Hawaii Ironman. Seeing this was over 30 years ago it was based on a hunch not on exact science. Being unable to measure core and skin temperature it was just get in, grin and bear it, jog on the spot occasionally before going a bit light headed, sit down and yes, rinse-and-repeat: grin and bear it a bit more.
Why could winter be a good time for heat?
So I do like the idea of using some heat in winter, not excessively though. Not accepting the “off” in off-season means pushing hard every month of the year, which is just not sustainable. Letting your mind and body have some ease-back means I would prescribe easier heat sessions now and the real peak-fitness building hard core programme 4–6 weeks before big races only. It’s pointless to be the King (or Queen) of December and lose motivation to progress strongly come the springtime build toward your key challenges.
Never get cold
Actually my second rule of coaching athletes is, “never get cold.” In fact, I have updated it to “… and make sure you get hot occasionally.” It makes no sense to dress in minimal clothing and train outdoors when the mercury drops low. I know there’s the hard core who think they are harder, smarter and better off dressing in minimal clothing, but they are wrong. Muppets in fact. Similarly some athletes put the same clothing on winter, summer, spring, rain, sun, frost or heat wave. Again, Muppets.
The best of the best do not try to get cold and train in misery. They adapt clothing to keep themselves warm to hot in training and pre-cool before racing. I will add there are clearly cold water swimmers, climbers, etc. who train for extremely cold conditions but you are not my audience right now. Brrrrrrrrrr.
Home saunas and hot baths
So having heat as part of your winter programme using a sauna works for my coaching mindset. Maybe just a very hot bath as a cheaper option. When is best? Well, after aerobic or strength training, or maybe on a non-training day as part of additional help to improve recovery and injury rehab. Take note: heat is the new recovery mindset for injuries and not old-skool ice application [1].
Mini home sauna tents are relatively cheap (£190, $250, €215), quick to heat up, keep you time-efficient, and you get to not share bugs with those who still insist that going to a public sauna to clear their cold is a good idea. If that’s not your thing, hot baths have some good science to suggest they also help improve fitness and injury recovery if you dunk in after training.
Does passive heat training impact fitness & performance?
You want to know that something works rather than just following someone’s total guess (maybe a bad guess) that actually makes you worse. Increases in injury rate, drops in fitness, wasting time that could be used for effective methods – these are not your goals, my friend. Therefore, enter the 2021 study by Nathalie Kirby and a team comprising UK and Canadian Sports Scientists – they have done the work for us [2] in an off-season time scale (Oct–Feb) real world piece of great science.
They studied runners who took 30-minute saunas after run sessions, 3 times a week for 3 weeks. If you read the abstract it says the dry sauna (5–9% relative humidity) was 101–108°C/215°F. However, that was at 1.8m off the ground; it was 86°C/187°F at the runners’ chest height and only 60°C/140°F at the seat height. Clearly where you sit in a sauna has a big impact on the extent to which you feel the heat.
Fast forward 3 weeks to the end of the study and the extra heat load of just 4.5 hours dry sauna had made quite a few statistically significant impacts on these runners:
- Resting Heart Rate (HR) dropped by 3 beats
- Highest HR during 30 mins hot running (40°C/104°F, 40% humidity, 9kmh, 2% grade) dropped 8–10 beats
- VO2max rose 0.12–0.15L/min (or 2.0–2.6 on the classic Garmin/Polar Style fitness metric (aka VO2 in ml/kg/min.))
- For the FTP lovers out there: this also rose by 2% based on accurate lactate analysis
The best part: both male and female runners took part and despite some assumptions that the researchers made about how the genders would differ after the passive heat training, the data actually showed both genders gained significant fitness (albeit with some technical nuances on how each gender adapted).
There is also a good two person case study that showed great effects of post-running saunas and the super keen just need to Google “Dahlquist 2023 sauna”.
However, I must add that a recent analysis of ten studies using hot water baths or saunas after exercise only showed a “trivial” impact [3]. The effort put in “probably” gives very little back but this may be the individual methods used. Therefore do not give up on the heat-after-training concept. I think some of this lack of impact is down to the exact methods and the timing of the heat stress.
The plan
So, exactly how and when do you add sauna sessions into your training and recovery time?
In Winter, unless you deliberately train with no fan, extra clothes and/or a deliberately super-hot room, you are unlikely to get aggressively hot (core temp >38.5°C with high skin temp). Again, I warn against doing maximal heat training (like hard HIIT sessions) too early in the winter phase – you are merely icing a cupcake instead of focussing on building a big “aerobic” fruit cake first, then add icing later in Spring.
I suggest to my athletes not to push heat training too hard in the winter: saunas or hot baths are okay, as you need them to increase mood and feel good factor – but do not endure really hard heat training trying to find the last 2–3% when races are often over 4–6 months away.
Even if you do add hard intervals and heat afterwards in a super cocktail, well sorry, the research analysis [4] suggests that “heat top-up” is probably not going to add anything extra to the hard work already amassed in the interval sessions with normal room temperatures afterwards. Save the bother.
Tracking sauna sessions in the CORE app
The science of training is all about measurement – it’s what I love about coaching and endurance sport. Well-defined training zones, measured carbohydrate intake, food energy analysis, tire rolling resistance, rates of sodium loss and so, so many other metrics all make training outcomes better by not using guesswork. Heat training is no different and the Core sensor brings the lab into your active heat training sessions and now passive heat training too via the Passive Heat Calculator.
By measuring data and using the CORE app you get a precise calculation, graphs to study and a heat adaptation outcome which has previously been a guess. To see and feel you are better in the heat is confidence inspiring, especially when many events start early in the day and temperature builds ever upwards as you struggle toward the finish line.
Remember to not wear the CORE sensor into the sauna/tub (data will be inaccurate, and the heat of the sauna may damage the unit). Instead, wear the sensor during your workout right up until you enter the sauna/tub – then the app will know your starting core temp when you enter the session into the Passive Calculator.
Heat is a good thing to use, even for those of lower level fitness who want better blood pressure regulation and to feel better about their health [5]. Whether a hard-core Ironman, a fitness cyclist, avid runner or a get-fitter, you can cleverly use heat to improve injury recovery, increase fitness and raise perceived fitness belief – a sensor that keeps you doing just the right amount of heat strain for your goals is a good thing. We thankfully don’t have to go back to the grin-and-bear-it heat sessions of yesteryear. Thank you science!
8 Tips To A Better Sauna Protocol
- Avoid if you have uncontrolled cardiovascular disease, acute illness, fever or syncope history. If unsure seek medical clearance, especially if on blood pressure/thermoregulatory medicines.
- Be hydrated by drinking roughly 500–700 mL with electrolytes in the hour before. Sip as needed during the sauna aiming to replace ~150% of body mass lost. Monitor your body mass change by weighing before, and after limit fluid loss ≤2% body mass.
- Time the nature of your heat training to fit in with your goals. If its early mid-winter and no immediate key races, then just use warmth for comfort by reducing the frequency, duration and heat of the sessions. If you are building to a fitness peak do 3–5 weeks of focused heat exertion (see #5 below).
- Stop or adapt if dizziness, nausea, headache, chills, confusion, or unusual heart beat occur. Prioritise safety over trying to maximise heat dose. If resting HR (morning), HRV or sleep are significantly changed, then adapt the next session. Simply listen to your body.
- Start saunas (or hot baths) within 10–15 min after key aerobic sessions or resistance training. For the peak heat intensity aim for 100–105°C dry sauna, seated, 25–30 min per session, 3×/week for 3–5 weeks (≈9-15 sessions). The target session RPE would be 6–8/10 so that resting (seated) HR is ~50–70% HRmax.
- Don’t break up the sauna or hot baths with cold showers. You can drink liquids during heat sessions – it is not about extreme thirst, dizziness or failing to complete the whole 3–5 weeks of peak heat stress. Consistency counts.
- Use the Passive Heat Calculator in the CORE app to plan your session so you get the Heat Training Load you’re looking for.
- DO NOT have saunas or hot baths before training – this will not improve performance.
References
[1] Dablainville, V. et al. (2025) Muscle regeneration is improved by hot water immersion but unchanged by cold following a simulated musculoskeletal injury in humans. J Physiol (1) pp 1–23.
[2] Kirby, N, Lucas, S, Cable, T, Armstrong, O, Weaver, S & Lucas, B (2021) Sex differences in adaptation to intermittent post-exercise sauna bathing in trained middle-distance runners. Sports Medicine - Open, vol. 7, no.1.
[3] Solomon, T.P.J. & Laye, M.J. (2025) The effect of post-exercise heat exposure (passive heat acclimation) on endurance exercise performance: a systematic review and meta analysis. BMC Sports Sci, Med. Rehab. 17:4.
[4] Kjertakov, M. et al (2025) No significant effects of supplementing high-intensity interval training with post-exercise hot water immersion on either temperate endurance performance or mitochondrial adaptations. Journal of Thermal Biology 131 (2025) 104119.
[5] Steward, C.J. et al (2025) Post-exercise hot water immersion enhances haemodynamic and vascular benefits of exercise without further improving cardiorespiratory fitness, glucose, lipids or inflammation. J Physiol. 603(16): pp4515-4533. doi: 10.1113/JP288873.
About The Author - Joe Beer
Coach Joe Beer has been in endurance sports since the mid 1980s; from 10k’s and half marathons came the Bath Triathlon and eventually Escape from Alcatraz Triathlon, The Ironman World Championship in Kona Hawaii to time trials, duathlons, London-to-Paris charity cycles, Ultra runs and most things in-between.
Joe uses his Health, Fitness and Performance model to help those wanting to lose weight or finish an Ironman. From professionals to fitness beginners he has helped thousands achieve their goals. He has written for magazines for over 30 years, has two books published plus hundreds of articles and blogs across the internet. Look out for his podcast on Spotify, Apple Podcasts and other platforms.
Joe practices what he preaches with aerobic sessions, heat training, weights, key workouts to power and the odd competition alongside his partner Debbie. There’s talk of a 100-mile run.
He is constantly learning how to be healthier, fitter and perform in challenges and loves passing experience onto others.