How useful is HRV tracking?

I haven't done it myself, but you could probably do an okay job of approximating lactate thresholds with HRV thresholds.

I find almost all the metrics that Garmin spits out at you useless, but HRV actually seems to correlate with my general well being. Most of the time it just tells you what you already know, but sometimes you can see the number move and use that info to up your recovery or what have you.

I wouldn't buy a new device for it though, just wear your watch.

I was was wondering how useful it is as well. Especially since most everyone says you should get a chest strap or armband HRM because optical sensors are not reliable on the wrist. Are HRV data any better?

Optical wrist sensors can be unreliable while running, but otherwise they're fine.

HRV is measured at rest overnight

I find it's quite well correlated with having had some kind of issue in the previous days but not a useful leading indicator. Usually a minor infection, disrupted sleep, heavy drinking etc. will show up in Garmin HRV once I'm starting to feel OK. Resting heart rate seems to be equally well correlated but responds quicker. In almost every case, I know there's something going on before it shows up on the watch (or before it happens, in the case of a big race or a late night party or whatever).

Sometimes it can give you a day's notice that you're getting sick before you notice yourself. Apart from that it's not super useful

Just use your watch tho

But you should still be able to feel the increase in HRV in bed. Here's a description of how it works for most of us.

Increased endurance training drives a rapid (within days) expansion of plasma volume (PV), often by 12–20%, resulting in a higher total blood volume. This expansion, known as hypervolemia, acts as a pivotal physiological adaptation that enhances cardiovascular function, leading to increased systolic filling and higher heart rate variability (HRV).

Systolic Filling and Cardiac Function
Increased Stroke Volume (SV): The expanded plasma volume increases venous return to the heart, leading to higher end-diastolic ventricular filling (preload). This allows the heart to pump more blood per beat (increased stroke volume).
Reduced Cardiac Effort: Because the stroke volume is higher due to greater filling, the heart can maintain the necessary cardiac output during exercise with a lower heart rate.
Enhanced Cardiac Output: Plasma expansion directly enables a higher maximum cardiac output, supporting better muscle oxygen delivery.
Increased Heart Rate Variability (HRV) [1, 2]
Increased Vagal Tone: Endurance training leads to structural remodeling (e.g., increased heart size) and electrical remodeling of the heart, resulting in a lower resting heart rate (bradycardia).
Parasympathetic Dominance: This lower heart rate is strongly associated with increased vagal (parasympathetic) modulation of the heart, which is measured as higher resting HRV.
Training Intensity: High-intensity endurance training is particularly effective at increasing nocturnal heart rate variability and vagal-related HRV indices (like RMSSD) in sedentary individuals, often correlating with improvements in VO2max.
Rapid Adaptation Mechanism
Plasma Volume Expansion: The expansion of plasma volume occurs quickly, often with a 12–20% increase in just 4–8 days of training.
Role of Albumin: The expansion is driven by increased total circulating plasma proteins (like albumin), which increases oncotic pressure, drawing more water into the vascular space.
Performance Benefits: This hypervolemia provides better thermoregulatory capacity and cardiovascular stability in subsequent exercise