The Stress Index (SI) is used as an indicator for our stress level. Its calculation is based on the variation of our heart beat intervals.
The SI is only significant if a person is organically healthy. Diseases, physical strain (like exercising) as well as psychological states like stress oder depression have a significant impact on our stress index.
If the SI is repeatedly high without apparent stress triggers, a high stress index might indicate health issues. You might want to see a doctor to evaluate your condition.