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.
