HRV, resting heart rate, sleep: What your readiness score really tells you

Almost every sports watch now shows a recovery or readiness value: a number in the morning that's supposed to tell you whether today is a good day for hard intervals. Behind it are usually three measurements: heart rate variability (HRV), resting heart rate and sleep. All three are well researched, and all three get routinely over-interpreted in everyday use. Time to sort that out.

What HRV actually measures

Your heart doesn't beat like a metronome. Between two beats there are sometimes 850, sometimes 920 milliseconds, and exactly that fluctuation is HRV. What's usually measured is RMSSD, during sleep or right after waking. High variability is considered a sign that the parasympathetic system, the recovery branch of your nervous system, is in charge. If HRV drops below your usual level, the body is under stress, whether from training, an infection or your job.

Two things are crucial here. First: HRV values are not comparable between people. Whether your training partner sits at 110 and you at 65 says nothing about your fitness, only the deviation from your own baseline counts. Second: single values fluctuate a lot from day to day. Research on elite athletes therefore recommends looking at rolling 7-day averages instead of individual mornings (Plews & Laursen 2013).

Low HRV: one signal, many causes

A depressed HRV only tells you that something is going on, not what. Common causes besides hard training load: an oncoming infection, alcohol the night before, heat, a late hard session, poor sleep, mental stress, and for women the menstrual cycle phase. And there's a known trap in the other direction: in highly endurance-trained athletes, HRV can paradoxically stay high or even rise in an overreached state, so-called parasympathetic saturation (Plews et al. 2013, Buchheit 2014). Rely on a single number and you can be wrong in both directions. Only the combination with resting heart rate, sleep and subjective feel makes the picture solid.

Resting heart rate: slow, but honest

Resting heart rate reacts more slowly than HRV but is more robust to measure. If your morning pulse sits clearly above your baseline for several days, as a rule of thumb about 5 beats, that's a signal worth taking seriously: incomplete recovery or an infection rolling in. A meta-analysis on overreaching found exactly this pattern: slightly elevated resting heart rate in overloaded states (Bosquet et al. 2008). Long term, the opposite direction is the good sign: as aerobic fitness grows, resting heart rate falls.

Sleep: the biggest lever you have

No wearable metric replaces the mundane truth that sleep is the most effective recovery tool. The consensus recommendation for athletes is at least 7 to 9 hours (Walsh et al. 2021). In one widely cited study, simply extending sleep over several weeks improved sprint times and precision performance in college athletes (Mah et al. 2011), while sleep restriction measurably worsens reaction time, mood and recovery (Fullagar et al. 2015). If your readiness value is chronically low, looking at sleep duration usually pays off more than any training adjustment.

What research says about HRV-guided training

The most interesting question is: does steering training by these values actually help? Several randomized studies tested HRV-guided training against rigid plans, scheduling hard sessions only when HRV was in its normal range (Kiviniemi et al. 2007, Vesterinen et al. 2016). The meta-analyses paint a sober but positive picture: effects on VO2max and performance are small but consistently in favor of HRV guidance, and above all there are noticeably fewer athletes who fail to respond to training at all (Granero-Gallegos et al. 2020, Manresa-Rocamora et al. 2021). Put differently: the value lies less in optimizing good days than in avoiding bad ones.

What a readiness score can and cannot do

A good score condenses these signals into a tendency and spares you the daily interpretation of raw data. But three limits always remain:

• Trend beats single day. One bad morning is noise, three bad days are a pattern.
• The score doesn't fully know your context. A glass of wine, a thunderstorm outside the bedroom, a stressful week: the number sees the effect, not the cause.
• The score is an advisor, not a boss. If you start living by the number and let a red value ruin your day, you've turned the tool against yourself. If you feel good and the score is mediocre, when in doubt your body wins.

How trace calculates readiness

In trace, readiness is a value from 0 to 100 that combines HRV, resting heart rate and sleep with your current training load, each measured against your personal baseline, not against population averages. You don't just see the number, you see every factor individually with its trend. And trace is honest about the limits: if measurements are missing, say because no watch delivers HRV, the platform tells you exactly that instead of claiming a physiological recovery it cannot measure. The data flows in automatically via Apple Health, a connected watch or intervals.icu.

The bottom line: HRV, resting heart rate and sleep are real, well-documented signals, not wellness folklore. But they are listening tools. They don't replace your feel for your own body, they sharpen it.

Sources

• Plews & Laursen et al. (2013): Training adaptation and heart rate variability in elite athletes, Sports Medicine
• Buchheit (2014): Monitoring training status with HR measures, Frontiers in Physiology
• Bosquet et al. (2008): Is heart rate a convenient tool to monitor over-reaching?, British Journal of Sports Medicine
• Walsh et al. (2021): Sleep and the athlete, British Journal of Sports Medicine
• Mah et al. (2011): Sleep extension and athletic performance, Sleep
• Fullagar et al. (2015): Sleep and athletic performance, Sports Medicine
• Kiviniemi et al. (2007), Vesterinen et al. (2016): HRV-guided training, EJAP / Scand J Med Sci Sports
• Granero-Gallegos et al. (2020), Manresa-Rocamora et al. (2021): meta-analyses on HRV-guided training