People with higher heart rate variability tend to have more activity in a part of the brain that helps calm stress and fear, which might mean their body is better at handling stressful situations.

When people feel stressed, their heart doesn't relax as well between beats, and this change in heart rhythm might be a simple way to tell if their body's stress system is out of balance.

Your heart's rhythm changes based on things like how old you are, whether you smoke, how fit you are, or how well you slept — so if you're using heart rhythm to measure stress, you need to take all these other factors into account.