For building speed, strength, and power in one bite-sized workout session, you can’t beat high-intensity interval training (HIIT). Those short and sweet sessions pack a punch, boosting metabolism, improving speed and endurance, and overall fitness in just 10 to 15 minutes.

But like any other type of strenuous exercise, HIIT stresses your body, which can negatively impact hormones like cortisol when done too frequently or without proper recovery. Here, experts share how to optimize those HIIT sessions for maximum fitness gains—and when it’s time to take it easy.

First things first: What is HIIT?

Like its name suggests, high-intensity interval training is any exercise that alternates between short intervals of intense activity followed by brief periods of rest or active recovery. Done at your ideal heart rate—usually 70 to 90 percent of max—these high-intensity intervals get the heart rate up quickly, boosting metabolic efficiency while improving cardiovascular fitness and muscular strength, says Julia Caban, CPT, a certified personal trainer based in San Diego.

HIIT is a staple workout at popular national fitness chains like Barry’s and Orangetheory Fitness, but can be incorporated into any kind of physical activity— think cycling, circuit training, or running. And unlike steady-state workouts—prolonged cardio sessions of 50 to 60 percent of your maximum heart rate done for 30 minutes or longer—HIIT can be done in as little as 10-minute sessions, giving you maximum fitness gains in a minimal amount of time. Plus, HIIT is a great way to mix in fun, varied movements like sprint intervals, jumping jacks, and stair climbing into your usual workout routine.

Okay, so what is cortisol?

According to Sharon Horesh Bergquist, MD, an internal medicine physician in Atlanta, cortisol is a hormone that plays a key role in the body’s stress response. “When the body senses danger, the brain signals the adrenal glands to make cortisol, which mobilizes the body to respond to that threat,” she explains. Dr. Bergquist says intense exercise like HIIT can increase the body’s cortisol levels, but so can lifestyle factors like illness, infection, pregnancy, sleep deprivation, and certain medications like steroids.

When performed in short bursts, intense physical activities like HIIT spike cortisol levels, but “provide a good kind of physiological stress, as it makes our bodies more resilient and adaptable to stress over time,” Caban explains. That adaptability means your body and mind will be prepared to handle everyday stressors, whether that’s tackling a challenging work presentation or powering up a steep hill during a weekend hike.

What’s the connection between HIIT and cortisol?

Like any type of high heart rate exercise, HIIT increases cortisol because “the body perceives the activity as a form of stress,” Dr. Bergquist says. “Generally, the more intense and longer the intervals, the greater the increase in cortisol,” she continues.

But that’s not necessarily a bad thing, says personal trainer Hannah Davis, CPT, CSCS, owner of Body By Hannah in Cleveland, Tennessee. “These exercise-induced cortisol spikes actually improve metabolism in the long run, which is good for overall hormonal regulation and stress management,” she explains.

Dr. Bergquist agrees. “Unlike chronic stress, the surge in cortisol from HIIT is brief, typically peaking at about 30 minutes and returning to baseline within two hours post-workout,” she explains. “This brief, intermittent stress triggers a healthy, adaptive response in the body.”

In other words, these temporary spikes in cortisol make your body more fit and resilient over time—and can lower baseline cortisol levels because HIIT teaches your body (and your brain) how to process physical and psychological stress.

Signs your cortisol levels are high

According to Dr. Bergquist, intense exercise without proper recovery, sleep, or diet can cause raised cortisol levels. So can prolonged periods of mental stress, anxiety, or illness. That stress manifests in the body via symptoms like weight gain, abdominal stretch marks, increased blood sugar, and high blood pressure.

Medically, a “normal” cortisol range is 5 to 25 mcg/dL (micrograms per deciliter) when measured via a traditional blood test. “Being outside the normal range cortisol of the lab reference level typically indicates specific issues like Cushing’s disease or adrenal conditions,” Dr. Bergquist explains. So if your cortisol levels are higher than the norm, she recommends consulting a doctor for the appropriate diagnosis and treatment plan.

But Dr. Bergquist says current medical blood tests only provide a snapshot of cortisol at a single point in time. “For chronic stress, what really matters is measuring the cumulative exposure to high cortisol, which we currently lack the ability to measure directly,” she explains. So as with any type of exercise program, it’s best to consult a physician before starting HIIT or if you experience any symptoms of high cortisol.

When done properly and supported by optimal recovery, a regular HIIT practice is one of the best forms of exercise.

Who’s most prone to being affected by increased cortisol levels when they do HIIT?

For most healthy individuals, HIIT temporarily spikes cortisol, which then decreases after a period of rest. But for people newer to HIIT or cardiovascular exercise in general, those cortisol levels may stay higher for longer periods of time. The same goes for people who experience chronic stress, lack of sleep, and have diets higher in sugar and caffeine because “their stress systems are already activated,” Dr. Bergquist says.

“For people with high cortisol, I recommend starting with low to moderate-intensity exercises like yoga, resistance training, and walking, and then gradually building up to higher intensity.” Once you’ve built basic aerobic conditioning, “your cortisol will spike less during HIIT,” she continues.

At the same time, she suggests doubling down on strategic recovery practices, like getting seven to nine hours of sleep per night, consuming enough protein to support muscle growth, eating quality carbohydrates to replenish glycogen stores and plants high in phytochemicals (chemicals produced by plants that may prevent cancer causing agents from forming) to lower inflammation, and replenishing hydration and electrolytes.

“Light-intensity activities like walking or yoga can also speed recovery,” she says.

How do you know how much HIIT to do?

How many times per week of HIIT

HIIT can be a fun, heart rate-boosting workout, but it works best as part of a well-rounded fitness program that includes active recovery, steady-state cardio, and weight training, Davis says.

Dr. Berquist recommends that individuals with normal cortisol levels do HIIT at least once and no more than three times per week. Caban says as a general rule, HIIT workouts should comprise no more than 20 percent of overall weekly cardiovascular exercise. “This volume will allow you to experience the positive benefits of HIIT without risking overtraining,” she says.

How many minutes per HIIT session

One of the perks of HIIT is that it’s a highly effective workout that can be performed in as little as 10 minutes—no equipment needed. Caban says HIIT workout lengths can vary depending on an individual’s fitness level and goals, but 20 to 30 minutes is the sweet spot for building strength and cardiovascular endurance.

How to recover

When it comes to HIIT, more isn’t necessarily better. “Most people should avoid doing HIIT on back-to-back days to allow for proper recovery, especially because it can take up to 24 hours for cortisol levels to drop back to baseline after a session,” Davis says. Recovery time is crucial for repairing muscles and reaping the benefits that make you “physically stronger and more stress-resilient over time,” Dr Bergquist says.

To maximize recovery, Caban recommends prioritizing protein intake (a minimum of 0.8g per pound of bodyweight), getting eight to nine hours of sleep per night, staying hydrated, and eating enough calories to support training.

“HIIT unfortunately has gotten a bad rep, and unfortunately, individuals who could really benefit from HIIT in the right amount are avoiding it altogether,” Davis says. She suggests using a fitness tracker or smartwatch to monitor your heart rate during high-intensity activities to ensure you’re reaching the proper intensity zones while honoring prescribed rest intervals.

“Individuals who are negatively impacted by HIIT are more than likely not doing HIIT at all, but doing high-volume intensity training—more reps with lighter resistance—without proper recovery,” Davis continues.

But when done properly and supported by optimal recovery, a regular HIIT practice is one of the best forms of exercise for building strength and endurance, but improving overall hormonal—and mental—health.