How long until my HRV recovers after an easy session?

Stanley, Peake and Buchheit reviewed how quickly the heart's parasympathetic (vagal) control returns after exercise, as a marker of cardiovascular recovery, and analysed published data on aerobic exercise quantitatively by intensity, duration and fitness.

Their abstract concludes that complete cardiac autonomic recovery after one aerobic session takes up to 24 h after low-intensity exercise, 24-48 h after threshold-intensity exercise and at least 48 h after high-intensity exercise; that, on limited data, exercise duration is unlikely to be the greatest determinant; and that people with greater aerobic fitness recover faster.

They propose that the metaboreflex (muscle and blood acidosis) drives parasympathetic reactivation in the short term (0-90 min) and the baroreflex (exercise-induced changes in plasma volume) in the intermediate term (1-48 h), and note that cardiac parasympathetic reactivation does not appear to coincide with recovery of all systems, such as energy stores or the neuromuscular system — though data after strength exercise are limited.

They argue that cardiac parasympathetic data, together with training logs, help individualise training, and give microcycle recommendations (not detailed in the abstract).

Our reading: the ordering low < threshold < high is the useful part for the base; it agrees with REF-161 that easy work disturbs the autonomic system least and that duration matters less than intensity, but its "up to 24 h" for low intensity is a different endpoint and time window from REF-161's "back to baseline in 5-10 min" (supine HRV over a 4-h window), and above VT1 the two disagree: REF-161 found threshold and above-VT2 sessions recovered alike (~30 min), while this review orders threshold (24-48 h) below high intensity (at least 48 h), with bands the abstract does not define; so "easy is cheap" means cheapest, not invisible the next morning.