High-Intensity Interval Training (HIIT) has become one of the most talked-about fitness methods in recent years, and for good reason! It’s fast, effective, and backed by research showing benefits for cardiovascular health, fat-burning, and even brain performance.

Chelsea Brubeck, NASM Certified Personal Trainer and Certified Nutrition Coach, breaks down what HIIT really is, how it works, and how to get the most from it—especially when it comes to balancing performance with recovery.

What Is HIIT?

“HIIT, or high-intensity interval training, is defined as a training method of intervals of the near max and max intensity levels we hit in that zone 4–5 area, broken up by short rest periods throughout the session,” explains Brubeck.

The idea is to cycle between short bursts of maximum effort, such as a 30-second sprint, and brief recovery periods, like walking for 90 seconds, then repeating for 15 to 30 minutes. The goal is to push your heart rate into the higher training zones (4–5), then allow it to come back down before ramping up again.

Benefits of HIIT: Efficiency Meets Performance

HIIT sessions burn more calories in less time than steady-state cardio like Zone 2. These workouts can often be completed in under 30 minutes, sometimes even as little as 15 minutes, making them ideal for busy schedules.

One of the biggest physiological advantages? HIIT significantly improves VO₂ max—your body’s ability to take in and use oxygen during intense exercise.

Higher VO₂ max means:

More oxygen delivered to your muscles

Greater energy output

Reduced strain on the heart

Long-term cardiovascular health benefits

“HIIT sessions are probably one of the most beneficial forms of exercise,” Brubeck says. “Not only because of everything we just mentioned, but also because of how little time it actually can take.”

HIIT and Cortisol: Friend or Foe?

Yes, HIIT spikes cortisol, the body’s main stress hormone. But according to Brubeck, that’s not necessarily a bad thing.

“Cortisol is responsible for triggering the breakdown of fat and production of glucose to create energy,” she explains. “Without it, our body would simply lack the ability to create this energy needed to perform these workouts or even just day-to-day functions.”

Short-term cortisol spikes are normal and even necessary during exercise. However, it’s important to monitor your post-workout recovery. If cortisol levels remain elevated for too long, this can lead to:

Poor sleep

Brain fog

Belly fat storage

Slower recovery

Brubeck recommends paying attention to how your body feels in the hours following a HIIT session. Is your heart rate returning to normal? Are you feeling mentally sharp, or exhausted and foggy? “It is important to listen to your body in these down periods,” she says. “The recovery period is truly when we are able to understand our bodily responses to these physical stressors.”

Start Smart: HIIT Isn’t One-Size-Fits-All

As effective as HIIT is, Brubeck cautions against diving in too aggressively, especially for beginners or older adults.

“There’s rarely a benefit to jumping headfirst into anything,” she notes. “With a decent amount of room for possible injury, it’s easy to get ahead of yourself.”

Her advice:

Talk to a certified trainer

Get your max heart rate measured, if possible

Watch for warning signs like dizziness, nausea, or a heart rate that doesn’t normalize after resting

“These are all signs that your body is trying to communicate with you,” says Brubeck. “You should listen to them.”

The Takeaway

HIIT is one of the most efficient ways to burn fat, build endurance, and improve your heart and brain health—all in less time than a traditional workout. With sessions as short as 15 minutes, it’s a versatile option you can fit into your morning routine, lunch break, or evening wind-down.

But with great intensity comes great responsibility. Listening to your body, recovering well, and easing into it, especially if you’re new, are essential steps for staying healthy and avoiding burnout or injury.

Whether you're aiming to improve cardiovascular fitness, increase energy, or support mental clarity, HIIT can be a powerful tool, as long as it's done mindfully.

To learn more about the benefits of HIIT, watch our full conversation with Chelsea Brubeck in the video above.

By Alicia Barber Minteer, PhD