Complete Health, Inc.
HRV, or heart rate variability, is one of, if not the most accurate methods to measure day to day stress and stress over time. It specifically gives you objective measurement of your autonomic nervous system balance (parasympathetic vs sympathetic). HRV is also an excellent overall health measure as it represents the body's ability to adapt generally. After much research, I'm convinced the best device to measure HRV is called the Corsense and the App is called Elite HRV. You can purchase the Corsense here.
Here is an excellent general discussion about the usefulness of HRV from the Harvard Health Blog: