A better sleep score did not mean better recovery

Last night's sleep score came in at 77, 24.8% above its 7-night average of 61.7. Scroll past the headline number on most sleep trackers and that is where the story would usually end: a strong number, a quiet night, nothing more to check.

A different number told the other half

Overnight heart rate variability measured 74.0 ms, 6.2% below its own 7-night average of 78.9 ms. Resting heart rate came in at 52 bpm, 3.9% above its 14-night average of 50.1 bpm, and on a 14-night window HRV itself sat 9.5% below its own average there too. Readiness that morning landed at 47, a number the plan read as low enough to move a hard interval session off the day, toward a day expected to recover better. Three numbers from the same night, and only one of them agreed with the sleep score.

The sleep score and the readiness number were reading the same night and landing in different places. One said the night went well. The other said the body was still working through something the sleep score alone could not see.

This is not a story about which number was wrong. Sleep score, HRV and resting heart rate each measure something different: time and depth of sleep for one, the balance of the autonomic nervous system for the other two. A night can score well on one axis and poorly on another, and the gap between them is itself useful information.

Why one reading can mislead

¹ A review of HRV monitoring in elite endurance athletes found that both rises and falls in HRV have been linked to negative adaptation, which means a single night's reading cannot sort a clean recovery from a rough one on its own. The review is a narrative synthesis built from training studies in elite athletes. It argues for averaging HRV across days and reading it against each athlete's own baseline built over time. It gives no fixed number for how low counts as too low; each athlete's baseline is learned individually.

² A four-week study of 26 moderately fit men tested something close to that idea directly. Over 4 weeks, the group whose running training was set by daily HRV readings raised VO2peak from 56 to 60 ml/kg/min (P = 0.002), while the group following a predefined training plan moved from 54 to 55 ml/kg/min, a change that was not significant. The overall VO2peak gains of the two groups did not differ significantly from each other. What stands out in the method: the HRV group decided when to ease off using a reference value built from a 10-day rolling mean and spread, and from a 2-day falling trend, never from one morning's reading in isolation. The study ran only 4 weeks with 8 to 9 men per group, so it says little about what holds over a season.

³ A later 8-week study of 40 recreational runners followed the same logic over a longer block. The group training by HRV ran fewer moderate and high-intensity sessions than the group on a predefined plan (13.2 versus 17.7 sessions, P = 0.021), picking more easy days when the signal called for them. Their 3000-meter time still improved by 2.1% (P = 0.004), compared with a 1.1% change in the predefined group that was not significant. The authors describe the difference between the two groups as small, and both groups improved their VO2max over the 8 weeks: 3.7% in one group and 5.0% in the other.

⁴ A meta-analysis pooling 8 smaller studies and 199 participants, published between 2007 and 2020, found HRV-guided training gave a significant edge for vagal-related HRV measures, with an effect size of 0.50, over predefined training. It did not find a significant edge for resting heart rate, maximal aerobic capacity or endurance performance on their own. The authors describe any fitness or performance advantage as small given the evidence collected so far. They also say that choosing an HRV baseline, including which index and which averaging window to use, is still being worked out.

What to check before trusting one score

• Check HRV and resting heart rate alongside the sleep score.
• Compare each number to its own 7-night average, since any single night moves around on its own.
• See a mismatch between the numbers as a reason to ease off.

Why would HRV drop on a night the sleep score went up?

The data behind this story does not record a cause for this particular night, and no single reading settles whether a dip like this is a true setback or an ordinary swing. The research above points to watching the 14-night average and the recent trend to tell the two apart, since one night's number swings on its own.