Dr. Alan Watkins - Being Brilliant Every Single Day

If you are under pressure and your Heart Rate Variability is too high, your body goes into fight-or-flight mode and shuts down the brain's frontal lobe, which handles sophisticated thinking. This leads to poor thinking and hence to poor performance.

Although this is an unconscious automatic process, you can control it. You have to notice such stressful situations and control your breathing rhythm. You want a smooth rhythmic breath with a fixed ratio of inhale to exhale (e.g. 4sec in, 6sec out). This regulates your HRV, making your brain's frontal lobe work better and helping you think better.

Your Heart Rate Variability is the one that affects your performance, not your Heart Rate. You can perform well both with a high HR and a low HR, as long as your HRV is optimal. However, if your HRV gets too high, your heart rate is erratic and your performance suffers.

Your HRV depends on the emotional state you're in. Positive emotions such as excitement and contentment have an optimal HRV and thus enhance your performance, whereas negative emotions like anger and apathy have an abnormally high HRV and impair your performance.