As you have learned in our free guide, many factors affect Heart Rate Variability. The following answer assumes that you are taking readings back to back in a resting position (such as Morning Readiness readings).
The most relevant factors for differing back to back readings include structural stressors, breathing patterns and mental/emotional state. Let’s break this down.
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 to 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 are 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.
So Many Factors, How Can It Be Reliable?
As you can see, in a resting position back to back readings can vary quite widely. This is the reason that we look at trends over time (days, weeks, months) and we focus on repeatable methods for measuring Morning Readiness readings.
In a series of back to back readings, the first reading is always the most relevant. It is the most comparable to the first reading on other mornings, and it eliminates as many of the factors listed above (or at least ensures that they are as comparable as possible).