If you're not sure whether your body is ready for a hard run or needs an easier effort, you may want to check your stress score. When you're rested and fresh, you can handle the demands of harder training more easily. But if you're tired or on the verge of overtraining, the same hard workout can have the opposite effect. Your stress score is calculated during a three-minute test in which your HRV is analysed. The resulting stress score is shown as a number from 0 to 100, where a lower number means a lower stress level. This metric helps you judge what kind of activity your body is ready for. You'll get more accurate results if you take the test every day at the same time and under the same conditions (ideally before training, not after). This also helps you get a feel for how your results vary from day to day and week to week.
To take the HRV stress test, you need to stand up, because this makes the test more sensitive to low and moderate stress levels. When lying down, moderate stress indicators may not show up, but when standing, your cardiovascular system is under slight load. This load causes a noticeable drop in heart rate variability when your stress level is moderate rather than very low.
