High intensity interval training can produce amazing results. But, like anything, it’s best in moderation.

High intensity interval training (HIIT) has gained acclaim as being an efficient way to improve many aspects of physical fitness.

But without proper recovery, intense exercise can lead to elevated levels of cortisol in the bloodstream and heightened symptoms of physical stress, even when exercise is not being performed.

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Whether you’re on your Peloton bike, doing a YouTube workout, or attending a class at the gym, chances are good that you’ve heard of, and maybe even tried, HIIT.

A HIIT workout consists of short bouts of intense work lasting anywhere from 10 to 60 seconds, followed immediately by a period of active recovery of the same length or longer.

This cycle of hard work and recovery is repeated anywhere from 3 to 10 times, depending on the workout.

Positive physiological benefits from HIIT include heightened post-exercise metabolism, improved body composition, and improved fasting blood glucose and insulin sensitivity (1, 2, 3).

Because of the benefits gained within only a few workouts, HIIT has gained a reputation as being a “magic pill” of exercise.

With HIIT, you may see changes within a matter of weeks and walk away from your workouts feeling a new level of productivity and power.

It only takes a few intervals to experience your body’s heightened level of energy, which is influenced by a fluctuation of hormones, especially cortisol (4).

Cortisol is one of the many hormones our body produces to handle stress.

During HIIT, the brain senses stress, and a cascade of hormones is released, including cortisol. The release of cortisol activates the sympathetic nervous system, generating a fight-or-flight response (4, 5).

Historically, this sympathetic nervous system response to danger was the key to our early survival, providing our bodies with immediate energy and power to fight or flee when necessary.

Cortisol is responsible for physiological changes, such as the quick breakdown of fats and carbohydrates and a rise in blood sugar for immediate energy, and repressing the immune system to focus the bodies’ energy on the potentially life threatening task at hand (6).

Part of what makes HIIT training so effective at turning the body into a lean, fast, and powerful machine is this cortisol response that it generates (4).

As your legs start pedaling as fast as possible, your brain receives the message that your survival depends on this interval, at which point cortisol and other hormones are released, sending you into the sympathetic nervous system response.

The body then makes metabolic improvements following this energetically and hormonally demanding experience (2).

The problem with cortisol is that when our body has too much of it — either because of physical or psychological stress — it floats freely in the bloodstream, causing negative symptoms to creep into your everyday life.

Overtraining syndrome has some physiological causes, which may include a raised level of cortisol (7). The symptoms of overtraining syndrome include (8):

  • chronic fatigue
  • muscle fatigue or a noticeable decrease in power while exercising
  • changes in mood
  • lack of physical and psychological motivation
  • changes in sleep patterns or sleeplessness
  • feelings of anxiety
  • repressed immune system and consistent illness

When your body is overly taxed by an imbalance of cortisol, any of these symptoms can be present, even when you haven’t worked out within the last few days.

Ideally, your body should be able to accurately determine when the reaction of fight or flight is most useful and appropriate. But too much HIIT can confuse the brain into signaling a protective response even when our bodies are supposed to be calm or at rest.

Everyday tasks, such as packing lunches and driving to work, might leave you feeling agitated because your body is misinterpreting everyday stress as life threatening stress.

Because HIIT solicits such a powerful reaction from our sympathetic nervous system, it’s critically important to prioritize recovery when your workouts are frequently of high intensity.

In contrast to the sympathetic nervous system, the parasympathetic nervous system is responsible for sending the body into rest, digest, and recovery mode (5).

Recovery between intervals and recovery days between workouts are key to seeing positive physical results from your HIIT workouts (9).

The quality of your recovery is also important, and can be enhanced with different practices, including (9, 10, 11):

  • sleep
  • good nutrition and hydration
  • meditation and slow breathing exercises
  • foam rolling or massage
  • abstaining from intense exercise

If your body is constantly in a state of stress, the positive effects of HIIT can be reversed, with your hard work working against you.

Recognize your body’s state of stress, both psychological and physical, and if you feel any of the warning symptoms listed, take some additional time away from HIIT.

It is important to note that this kind of workout should be performed 2–3 days a week at most, with rest days between each HIIT session.

Periodizing your workout program on a monthly cycle is a good way to prevent harmful symptoms of overtraining, leaving yourself a few days that don’t include HIIT workouts (12).

While HIIT will make your body stronger in many ways, because of the cortisol response it generates, it is perceived by the body as stress.

The role of recovery is essential to maintain the benefits of HIIT workouts, as is an awareness of physical and psychological signs of chronic stress. Otherwise, your efforts may backfire.

So, next time you challenge yourself with a HIIT workout, be sure to plan for rest afterward to reap the biggest gains.

Alexandra Rose began her career in New York as a professional modern dancer and a personal trainer. After receiving a master’s in exercise physiology from Columbia University, Alexandra has worked within clinical exercise settings, commercial gyms, with pre-professional dancers, and with clients in their homes. Alexandra is a certified personal trainer and fascial stretch practitioner, providing clients of all athletic endeavors with body work that helps to restore healthy movement patterns, improve strength and performance, and prevent overuse injuries.