A: One of the most important things to understand about HRV is that it reflects many different stressors in a single number. Aside from your training, HRV is affected by everything from lack of sleep to mental stress to being sick and it’s not always easily predictable and can move both up or down depending on acute stress and accumulated stress.
It’s also worth noting that HRV is variable because the human body is dynamic, constantly changing and adapting. It’s perfectly normal to see fluctuations in HRV just as you’d see day to day fluctuations in resting HR, in blood pressure, in various hormone levels, etc. Just because your HRV went up when you expected it to go down, or vice versa, doesn’t mean HRV is not accurate or reliable, it simply means that there are a lot of variables that impact it and the human body is rarely completely predictable. Please read the rest of this book for more info.