HIIT is well-regarded as a game-changer when it comes to building strength and fitness, and now studies show it can have a remarkable impact on your sleep, mood and energy too. Dr Jinger Gottschall, Associate Professor of Kinesiology at Penn State University, explains how to best leverage HIIT for all around physical and mental health.
Emma Hogan: You recently led a ground-breaking study that highlighted how too much HIIT can lead to a plateau or drop in training progress. This study has also revealed some other interesting findings about how it influences sleep, mood and energy. Please tell us more.
Dr Jinger Gottschall: Yes! The research highlighted that if you exercise around 8-10 hours per week, spending any longer than 30-40 minutes a week with your heart rate above the 90 percent maximum training zone can negatively influence your exercise performance – causing a plateau or drop in training progress. But what was particularly interesting is how this optimal dosage correlates with this wider health indicators. Basically, those who have their heart rate in the 90 percent maximum zone for the optimal amount of time each week benefit from optimal sleep patterns, positive mood, and energy levels.
So what is the optimal amount of HIIT for quality sleep?
The survey data shows that sleep quality is higher when individuals spend this 30-40 minutes a week with their heart rate in the 90-100 percent maximum training zone. If individuals spend less time in that zone, sleep quality was lower, and if individuals spend more time in that zone, sleep quality was lower. The same can be said about mood too.
How do you know this? What is it that goes on in your body when you do a HIIT workout that will influence your sleep and mood in the hours that follow?
For this study, we focused on cortisol. We know that cortisol is a good thing in bursts as it helps the body repair, adapt and grow stronger. Intervals in a LES MILLS GRIT™ or LES MILLS SPRINT™ workout are great for driving these short-term increases. However, individuals who train intensively too frequently tend to have a baseline cortisol level that’s very high and therefore they don’t have fluctuations of a significant magnitude. These relatively flat cortisol levels are associated with poor sleep and poor mood and they even affect appetite.
In terms of the mood variables, too much cortisol is also related to increased fatigue, increased depression, low confidence. It’s important to note that cortisol can also be elevated due to non-exercised induced stresses. So if you’re just stressed due to daily life, it can also contribute to consistently elevated cortisol.
So this study measured individual’s cortisol levels and how the relate to sleep and mood. How did you actually gauge sleep and mood?
Sleep is easy to measure. We were able to get a really good sleep assessment using a Polar A370 heart rate device, which provides a total sleep time and a sleep quality score. We used surveys to measure mood and energy, focusing on the specific feeling states; esteem, vigour, tension, confusion.
The study was based on people doing a minimum of eight hours of exercise a week. What if you’re doing less (or more) than that each week, should you still aim to have your heart rate in the 90-100 percent maximum zone for a total of 30-40 minutes a week?
The critical measure really is the percentage of total exercise time in the 90-100 percent maximum heart rate zone – and the study shows that between 4 to 9 percent of total exercise time in this zone is optimal. If you’re doing the recommended eight hours of exercise a week, this is 30-40 minutes a week. Ideally you want to ensure that no more than 10 percent of your total workout pushes your heart rate into the 90-100 maximum zone.
Given that HIIT workouts such as LES MILLS GRIT and LES MILLS SPRINT are 30 minutes how many times do you recommend people do these workouts each week?
It’s important to note that sending your heart rate into the 90 percent heart rate zone is extreme. During a workout like LES MILLS GRIT or LES MILLS SPRINT it’s likely that your heart rate will frequently spike to the 85 percent max zone, but only hit the 90 percent training zone for around 10 minutes. I recommend doing LES MILLS GRIT or LES MILLS SPRINT workouts twice a week, and 2-3 cardio peak training workouts such as RPM™ or BODYATTACK™ – as it is likely that you’ll also hit the 90 percent maximum training zone for at least a few minutes during each these cardio peak training workouts. These small doses of time in the extreme heart rate zone are just as valuable.
So this effect is not isolated to HIIT?
Not at all. You get increased cortisol from other types of exercise, but the HIIT effect makes it more apparent.
If just 4-9 percent of your total exercise should be in this high-intensity zone, which is worse – not enough or too much?
I would honestly say that it’s worse to do too much. Because you’re putting yourself at risk, not only in regard to low quality and low amounts of sleep and your mood etc. But also, just in terms of injury. Cortisol does some good things for us in terms of helping build muscle, but too much of it does the reverse and your injury risk is higher.
We know there are some people who can’t get enough of HIIT workouts and will be frustrated to learn that there is such a thing as too much. What do you say to them?
It is hard. We know HIIT is very appealing, and we don’t want to discourage people from doing the exercise they like but we want to educate them on what is ideal and what will improve fitness and daily satisfaction. Which is why it’s great to now have this study show that spending just 4-9 percent of total weekly workout time in the extreme heart rate zone is optimal.
It’s also important to educate those new to exercise. If you’re not currently physically active, starting off with 30-40 mins of HIIT is not a good idea. You do want to slowly build into the vigorous training range regularly, and workouts like RPM, BODYATTACK, BODYSTEP™ and BODYCOMBAT™ are great for this. Once you’ve done this, which I advise should take at least six months at a minimum, then you can start to incorporate HIIT.
You can learn more about the optimal dose of HIIT here.
WHAT YOU NEED TO KNOW:
- HIIT is transformative because it pushes your heart rate into high training zones for short periods of time.
- But too much time with your heart rate in the extreme zone mitigates the spikes in cortisol production and can cause a plateau or drop in training progress.
- It also negatively impacts sleep quality, mood and energy levels.
- Spending 4 to 9 percent of your total exercise time with your heart rate in the 90 percent maximum training zone is optimal.
- If you’re doing eight hours of exercise a week, this is 30-40 minutes in the extreme heart rate zone a week.
- Two 30-minute HIIT workouts and 2-3 cardio peak training workouts (such as RPM or BODYATTACK) should enable to reach the optimal amount of time in this extreme heart rate zone.
Jinger Gottschall, PhD, is an Associate Professor at Penn State University who has led numerous studies into the effectiveness of various exercise regimes. Working closely with Bryce Hastings, Les Mills Head of Research, Dr Gottschall regularly tests the effectiveness of LES MILLS programs, helping ensure that exercisers get the best results from the time they spend working out. In addition to her research roles, Dr Gottschall is also a Les Mills instructor and fitness studio operator.
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