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.

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.

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.

When people experience stress, their heart rate variability tends to drop in a specific way — this change is seen again and again in different people and situations, and it might be a key sign that the body’s stress response isn’t working right.

When you're stressed, your heart's rhythm changes in a way that shows your body is shifting into 'fight or flight' mode and relaxing less — this change is often used by scientists to measure stress in the body.

Athletes who are overtrained might have an imbalance in their nervous system, with their body being in 'rest mode' too much and not balancing properly with 'active mode'.