Why back-to-back readings produce different results
Heart Rate Variability (HRV) results can vary from reading to reading for several reasons. Normal examples include changes in your circadian rhythm and hormonal patterns throughout the day. But even taking readings back-to-back can produce different results (for example, if you take your Morning Readiness and then delete it and take it again), and why we do not recommend doing it.
Many situational factors can cause the 2nd (or 3rd, 4th, etc.) readings to be different. The amount of time you spend in the reading position, slight changes in position, the thought of a reading messing up, or even the annoyance at taking multiple readings in a row can all affect the outcome of the reading.
The First Test
You may be taking the reading too soon after waking. If you wake without an alarm, there is usually a cortisol release associated with waking up that usually fades a few minutes after waking. If you use an alarm, it may take a few minutes for your body to adjust from the resting state to the waking state.
In either case, waiting a few minutes after you wake up to take your Morning Readiness reading may provide more consistency in the process. Feel free to get up and take care of any non-stressful tasks while waiting to get your blood flowing so to speak. See below for more information.
The Second Test
Try this test – take a reading in a seated position with your back reclined against a relaxing chair or wall. Then take a reading sitting up straight without back support. The results will likely be quite different.
Furthermore, the app can be quite sensitive to small changes in an acute situation. So even staying in the reading position longer than normal could register as a wide swing in your readiness in the span of a few minutes.
There is a reason the gold standard for short, rested Heart Rate Variability measurements. The longer you spend in a “rested” position, the greater the chance of fidgeting or mental stimulation occurring. This is exacerbated by a propensity for impatience.
The reasons and reactions for different back-to-back reading results are different from person to person. Some people’s HRV goes up the longer they sit, and some people’s HRV goes down.
Knowing this, the readings that were performed under the most similar circumstances are the most comparable. It sounds strange, but your first reading on two different mornings can be more relevant to each other than two readings taken back to back, depending on the circumstances.
For the most precise Morning Readiness results, aim to follow the process consistently each time without worrying too much. Small variations that seem significant in a single reading typically don't significantly influence the long-term trends.
Some of the most relevant factors for differing back-to-back readings include structural stressors, breathing patterns, and mental/emotional states.
Mental and Emotional State
Why does heart rate elevate when one gets on stage for a public speaking engagement? Our mental and emotional states can have a profound impact on our physiology at any given moment.
Regarding back-to-back readings: Your expectations of different results, potential boredom, annoyance, or excitement can all affect the outcome in addition to the other variables. In our experience, these particular changes (like boredom and annoyance) start occurring a little after the initial reading begins.
This is one of the reasons for the short Morning Readiness length as well as why subsequent readings in the same position can get increasingly affected by mental and emotional state.
Breathing patterns have a large effect on HRV. You might have guessed at this point that breathing patterns change over time.
Breathing patterns are regulated by your body’s needs automatically unless conscious control is asserted. Two large influences on breathing patterns in a static resting position are structural stress over time and mental/emotional state.
In general, the more calm, comfortable, and parasympathetic you are, the longer and deeper your exhaling is. As you get more excited or uncomfortable, typically breath becomes more shallow – especially on the exhale.
The same is true for any other body metrics you're tracking. For example, take blood pressure - the longer you sit there and take your blood pressure, the lower and "better" it will get. If you're taking blood pressure daily, you should take it at the same time and in the same conditions (as much as possible) each day.