How Do 'Sleep' and 'Daily Activities' Change with Heat Acclimation Training? - Introducing a Notable Study on 8 Recreational Trainees -
Hello. In this article, I will introduce a study on an interesting topic: how 'training to get used to the heat (heat acclimation training)' affects our sleep and our small daily activities.
For those involved in exercise or training, sports enthusiasts, or those who know from experience that 'training in the summer is tough...', I am sure this will be of interest.
The title of the study is,
"Evidence that heat acclimation training may alter sleep and incidental activity"
(Evidence that heat acclimation training may affect sleep and daily activities)
It was published in the European Journal of Sport Science in 2022 and was conducted by a research team from Australia and Norway.
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■ Background: Why is 'heat' attracting attention?
There is a reason why 'heat' is attracting attention as a training environment.
In recent years, endurance athletes and those performing high-intensity training have been using a method called 'heat acclimation,' which aims for physical adaptation by 'deliberately training in a hot environment.'
This method has benefits. For example:
• Strengthening of cardiopulmonary function
• Improvement of sweating function
• Performance improvement (especially in hot race environments)
However, it is also said that heat makes thermoregulation difficult, which can lead to fatigue accumulation and delayed recovery.
In other words, heat acclimation training is **'effective, but also risky.'**
What this study focuses on, among those 'risks,' is the impact on 'off-time' outside of training hours, specifically:
・Sleep quality
・Light daily physical activity (incidental activity)
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■ Study Overview: Verified with 8 'recreational men'!
The subjects were 8 healthy men with exercise habits, with an average age of 26. Rather than being 'athletes,' they were at a general fitness enthusiast level (VO2peak average 49.3 mL/kg/min).
They performed training under the following two conditions:
[Condition 1] Heat Acclimation Training (HA)
• Room temperature: approx. 35°C, humidity 53%
• 60 minutes on a bicycle ergometer
• 5 consecutive days
[Condition 2] Control (warm environment)
• Room temperature: approx. 22°C, humidity 65%
• Conducted under the same conditions otherwise
These were conducted in a crossover design (both were experienced with an interval of more than one month).
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■ What and how was it measured?
In addition to the load during training (heart rate, body temperature, subjective fatigue), data was collected focusing on the following points.
Sleep
• Sleep efficiency (percentage of time actually asleep)
• Sleep latency (time to fall asleep)
• Wake after sleep onset (time spent awake during the night)
• Sleep duration (total hours slept)
• Subjective sleep quality (self-reported)
Daily activity (outside of training)
• Sedentary time
• Light activity
• Moderate & vigorous activity
These were continuously recorded using **wrist-worn accelerometers (actigraphs).** The amount of movement in daily life was also tracked thoroughly.
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■ Results: The impact of heat was indeed present!
Now for the results you are curious about.
① Physiological stress definitely increased
• During training in the heat, **heart rate, body temperature, and subjective fatigue (RPE)** increased significantly.
• Especially for the first 3 days, the **Physiological Strain Index (PSI)** was remarkably high.
In other words, the burden on the body clearly increased.
② Impact on sleep
• Sleep efficiency decreased by an average of 2.1%.
• Wake time after sleep onset increased by an average of 7.6 minutes.
• However, sleep duration itself and subjective quality did not change.
What is interesting here is that 'the person does not notice it, but the quality is actually dropping.'
In other words, the discrepancy between subjective evaluation and objective measurement is also an important discovery.
③ Impact on daily activity
• Sedentary time increased by an average of 35 minutes.
• Light activity decreased by an average of 18 minutes.
• There was no difference in moderate to vigorous activity.
Outside of training hours, the time spent 'sitting still and resting' increased.
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■ Discussion: Why did this happen?
The research team's discussion is as follows.
1. The body needed more 'recovery'
• Training in a hot environment makes it easier to accumulate fatigue than usual.
• As a result, daily activity is unconsciously reduced, and time spent sitting and resting increases.
2. Possibility that decreased sleep quality further worsens fatigue
• Once sleep becomes shallow, recovery becomes insufficient.
• This leads to a vicious cycle where fatigue and stress accumulate.
3. A pitfall that is not subjectively noticed
• Even if you think you 'slept well,' the quality has actually decreased.
• Sleep 'quality' and 'duration' are different things.
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■ Tips for practice: Pitfalls of 'heat' training
Let's summarize the practical points obtained from this study.
1. Heat acclimation requires a 'recovery mechanism'
• Since training in a hot environment is high-load, a solid recovery strategy (sleep, nutrition, rest) is essential.
• It is especially important to pay attention to sleep quality.
2. Be aware of changes in daily activity
• You should also pay attention to how you spend your time outside of training (standing up and moving, walking frequently, etc.).
• 'Active rest' can also lead to effective recovery.
3. Use objective data, not just self-evaluation
• Use sleep logs and activity tracker data to find the gap with your subjective 'I'm fine'.
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■ Future challenges and prospects
This study also has some limitations.
• Small sample size (8 people)
• Subjects were at a recreational level
• No comparison with a baseline
• Applicability to other groups such as women and the elderly is unknown
However, while recognizing these limitations, the finding that 'heat environments affect how you spend time after training and your sleep' provides great hints for future research and practical settings.
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■ Summary
This study suggests that while heat acclimation training is useful for improving performance, it may 'unbeknownst to you' lower sleep quality or reduce daily activity.
To build a body that can overcome the heat, it is not enough to just work hard in the heat; a comprehensive approach that includes the quality of recovery and how you spend your off-time is necessary.
For those who have goals like 'I want to get stronger' or 'I want to win a race,' I hope you value your **'power to rest' and 'power to perceive.'**
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※ Reference paper:
Osborne, J. O., Minett, G. M., Stewart, I. B., et al. (2022). Evidence that heat acclimation training may alter sleep and incidental activity. European Journal of Sport Science. https://doi.org/10.1080/17461391.2022.2124386