Heat Acclimation: When To Start and How Long It Takes

Written by:

Jason Koop

CTS Premier Ultrarunning Coach

Updated On

March 7, 2025

By Jason Koop,
Head Coach of CTS Ultrarunning
Author “Training Essentials for Ultrarunning”

Western States, Wasatch, and Badwater runners – and any other athlete training for a hot weather race – listen up. Start thinking about your heat training protocols now. Don’t wait for the last four weeks of training. Don’t meticulously plan out your longest long run (not that it matters all that much) and haphazardly shoehorn your heat training plan du jour. Think and plan now because, if you do it right, you stand to reap a lot of benefits from a well laid out heat acclimation plan.

When done correctly, heat acclimatization training can be a powerful training intervention for both hot and cool weather races. However, if you do it incorrectly you create the potential to set your training back and unwind all of your hard-earned progress. In my observation of ultrarunners over the last 25 years, it’s one of the most underappreciated and consistently misapplied interventions. A quick tour of social media reveals athletes layering up in several layers of clothing, using the sauna for several weeks at a time, and even wheeling treadmills into the sauna in ill-fated attempts to achieve superior heat acclimation gains.

As a primer, I’ve previously created this cheat sheet for heat acclimation. If you are unfamiliar with the different modes (sauna, hot water immersion, active acclimation, etc.) I encourage you to check out that article. As a follow-up, I’d like to take you through a real-world example, using my own training for the 2022 Cocodona 250, of how to balance training, rest, training camps, and a heat intervention.

When to Schedule Heat Training Protocols

There are two categories of heat training modalities: passive and active. Passive modalities include a hot water immersion bath or a dry sauna (and I prefer the latter for athletes). Active strategies rely on running in hot temperatures. Regardless of which you choose, realize the protocol results in added stress on top of your run training.

As a result, plan your heat training when your training load is not the highest. This way you are not compounding running stress and heat stress. Similarly, don’t plan it during a recovery phase because you want to remove as much stress as possible during a recovery phase. From a practical standpoint, I normally schedule heat training protocols during an athlete’s 2^nd week of a 3-4 week block. I feel this is a good time to layer on an additional stressor (heat) at a time where you can maintain the run training load as-is or even reduce it slightly.

How Long Does Heat Acclimation Take And Last?

Acclimation to heat stress is an acute adaption. Once you are exposed to heat, it takes days, not weeks to achieve a robust response. A fantastic summary of this is presented below from Périard et al. Note that all the individual aspects of heat acclimatation happen in the first 5-9 days. Then, they come to near full fruition in 8-10 days. Therefore, the sweet spot for days in the sauna or hot water immersion bath is between 6-10 days. Not 3 and not 20.

Figure 1- Adapgted from Périard et al 2015

Anyone who uses a sauna for a short period notices this phenomenon play out in real-time. It only takes a few consecutive sessions to see changes. Your tolerance at the same heat improves, you sweat earlier, and that puddle at your feet grows. Turns out, the inverse is also true. When you remove heat exposure you lose your adaptations by about 2.5% per day. The take-home here is you don’t need much. You can be surgical and use a short intervention period that reaps a lot of benefit in a short time.

Two-Phase Heat Acclimation Protocol

Over the past few years, I have adopted a two-phase heat acclimation protocol inspired by some of the real-world work of Iñigo Mujika. The protocol uses passive sauna exposure to elicit the heat adaptions in two distinct phases. The first phase is approximately 6 weeks out from the race. The last phase is the final 6-10 days leading into the race. Between the phases is a maintenance phase where you get in the sauna every 2^nd or 3^rd day. This essentially sustains the adaptations from the first round. Remember what I said earlier about heat acclimation and de-acclimation being acute? This is key here.

Although I’ve successfully used a single-phase protocol with many athletes, I always felt it was a bit of a dice roll. You inevitably introduce heat stress near the end of the training process, which can be tricky or problematic. Particularly with athletes that are new to the protocol, you never really know how they are going to adapt. Introducing an unknown intervention at the very last minute is playing with fire. Sometimes they handle the heat just fine and adapt well. Other times it’s too much and results in a frustrated, fried athlete just when they need confidence.

In addition to avoiding potential negatives associated with a single-phase protocol, there are several advantages of using a two-phase protocol:

1. You get a low-risk first run-through far enough out from your event that you can calibrate the second round. This allows you to adjust the duration of the individual sessions, the number of consecutive days, etc. For women, this might fluctuate even more depending on the phase of your cycle you are in.
2. You can find a sauna or hot water immersion bath that is convenient and works for you. If you don’t have the luxury of having a sauna in your house (how is this not standard in the US?) it’s a chunk of time to drive to a gym and back.
3. Positive adaptations from the second round compound on top of the first, resulting in a larger, more robust adaptation.

In The Real World

Moving from practice to the real world here’s how I set it up for my training before I ran the Cocodona 250. Although I had used numerous sauna training protocols in the past, it had been several years since the last one. As a result, I felt the safest approach was a two-phase approach so I could use lessons from the first to calibrate the second. Note: this example is from my own training, and I rarely present my own behaviors as advice. In this case, I would still apply the same principles with any athlete, just with subtle variations on the timing.

First, I knew I was going to build a 4-day training camp around a course recon hosted by Aravaipa Running. That 4-day block was the biggest block of training in the entire build-up (5.5-9 hours a day for four consecutive days). It served as the anchor for the rest of the calendar. All the other long runs and training interventions revolved around its orbit.

Next, because of the stress of the training camp, I knew I wanted it to create a break between the first and second rounds of sauna work. Essentially, I wanted to end the first phase and begin the maintenance phase just before that block. I kept that phase of training at a normal volume, careful not to increase the volume or the volume of intensity.

Finally, I scheduled the second sauna block during the final 14 days before the race, to coincide with the taper. This means training dramatically reduces to accommodate the stress of the sauna, and also to rest before the race.

Click image to enlarge.

Did it work? Yes! A two-phase heat acclimation approach offered a low-risk, adaptable approach to an often problematic, but necessary, training intervention. Even if you think you know how you are going to adapt to your tried-and-true method, I encourage you to think about utilizing a two-phase approach. At best you compound your heat training adaptations from the first round to the second. At worst (when done correctly) you are only out the time you spent in the sauna. Not a bad trade-off. And for the good Lord’s sake, please stop running around in your puffy jackets.