About two days for a marathon-length effort, and longer for an ultra. In 13 recreational runners who took a morning reading every day around a 64 km race, heart rate was up and RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD , SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN and high-frequency power HF power The variance of the heartbeat intervals that falls in the 0.15 to 0.40 Hz band, in ms²: the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power were down the morning after, and the vagal indices were back to baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline on day 2 of recovery (Fazackerley and colleagues, 2019). Their soreness and fatigue lasted until day 5. After a 100-mile race, 24-hour heart rate variability (HRV) in 28 runners had still not returned to baseline a week later (Paech and colleagues, 2021).
What the race studies found
A 64 km race. Thirteen runners (eight men, average age 37) measured HRV on waking for a week before and a week after the race, and filled in mood, wellbeing and soreness scales. The morning after, heart rate was higher and every HRV index HRV triangular index The number of NN intervals in a recording divided by the height of the tallest bar of their histogram. A unitless geometric measure of overall variability that a few bad beats barely move. More about HRV triangular index was lower, which the authors read as suppressed parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system activity. By the second morning of recovery, the parasympathetic-related indices had returned to baseline. On the fatigue and wellbeing scales the runners were not fully recovered until day 5, with muscle soreness prominent throughout.
A 100-mile race. Twenty-eight runners (25 men) wore a 24-hour Holter monitor a week before the race, straight after it, and after a week of recovery. Heart rate was about 19% higher straight after. A week later, SDNN was still about 7% below its starting value and full recovery had not occurred. Body mass, heart rate and HRV before the race did not predict finishing time, age showed only a weak link, and finishers and non-finishers looked alike on the heart measures. The authors concluded that HRV can show the state of recovery after such a race but cannot say how recovered the muscles and tendons are, which is what limits most runners after 100 miles.
The general rule. A 2013 review of aerobic exercise put full recovery of the heart’s parasympathetic control at up to 24 hours after easy sessions, 24 to 48 hours after threshold efforts and 48 hours or more after high-intensity ones, with fitter people recovering faster and duration mattering less than intensity (Stanley and colleagues, 2013). A marathon run near threshold pace sits at the upper end of the middle window; a race run to exhaustion sits in the last one. The training page has an interactive version of these time courses.
Why the nervous system recovers before the legs
A race pushes the sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system branch up and the vagal brake off for hours, and the first night’s sleep after it is often poor. By the second night, the autonomic side has largely settled, which is what the morning reading measures. Muscle damage, inflammation and glycogen stores recover on a slower clock that HRV does not see. That is why a normal reading two days after a marathon is good news about one system and says nothing about the others.
Some athletes see the opposite pattern after a big block or a race: a run of readings above their usual range. A 2021 meta-analysis found that weekly averaged HRV rose in functionally overreached athletes, while single readings did not show it (Manresa-Rocamora and colleagues, 2021). A high week with heavy legs and flat sessions is a reason for caution, not a green light. See why is my HRV suddenly higher than usual?
How to read your own week after a race
- Expect one or two low mornings. The day after is almost always down. A return inside your usual range by day 2 or 3 matches the studies.
- Give the legs their own timeline. Soreness, heavy legs and poor sleep lag the number by days. Run easy for a week or more after a marathon even when HRV says recovered.
- Watch heart rate too. A resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate still a few beats above usual a week on fits incomplete recovery. After the 100-mile race it was HRV, not heart rate, that was still below baseline a week later, so read the pair together. See HRV vs resting heart rate.
- Use the 7-day average. Single mornings bounce. The lesson on building your HRV baseline shows how to compare a race week with your usual range.
For the day-to-day decision, read should I train when my HRV is low?
When to talk to a doctor
Talk to a doctor if, in the days after a race, you have chest pain, fainting, a racing or irregular heartbeat, very dark urine, or a fever, or if your resting heart rate and HRV stay well outside your usual range for more than a week or two while you feel unwell. HRV cannot diagnose anything.