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).
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 is 5 minutes or less. 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.
The key to getting the most accurate Morning Readiness results is to try to replicate the process as closely as possible each time and not stress about it. The long-term trends are usually not meaningfully affected by these small changes that appear to have a large impact on a single reading.
If you believe that your results are not reliable due to mental stimulation or fidgeting, then it is recommended to use guided breathing (at a natural pace) to help calm the mind and relax for the short reading.
More information: https://hrvcourse.com/the-8-biggest-mistakes-made-when-measuring-hrv/
Some of the most relevant factors for differing back-to-back readings include structural stressors, breathing patterns, and mental/emotional state.
Structural Stressors
Depending on the position you take your reading in, your body goes through phases of structural comfort and discomfort (sometimes cyclically as you make small subconscious shifts) in holding that same position. Think of this in terms of doing an isometric exercise such as holding a plank. The first 10 seconds may seem easy, but the last 10 seconds are quite stressful, and occasionally throughout you may shift your weight for some temporary relief. If you’ve ever looked at heart rate from the beginning of a plank to the end, you will see quite a difference in most cases.
Knowing this, the longer you stay in a position the more structurally stressful it can be and it can cause changes in subsequent back-to-back HRV readings. In other words, if you sit for 2.5 minutes it is much different than sitting for 7-8 minutes (the span of 3 Morning Readiness readings). This particular variable is less relevant in a comfortable lying position, in which case the opposite may occur – one may get more comfortable as the position is maintained.
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 2-minute mark in most people.
This is one of the reasons for the 2.5-minute Morning Readiness length as well as why subsequent readings in the same position can get increasingly affected by mental and emotional state.
Breathing Patterns
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. A great test you can perform to see these effects is to take a 1-minute reading just looking out a window and breathing freely – then immediately take another reading in the same position but following the guided breathing circle in the app at a comfortably slow pace.
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 would take it at the same time and in the same conditions (as much as possible) each day.