Readiness: HRV, resting heart rate, sleep and how you feel

What HRV is measuring

The gap between consecutive heartbeats is never fixed. At rest it stretches and shortens with each breath, and the size of that fluctuation tracks how much vagal (parasympathetic) activity is shaping the rhythm. Heart rate variability puts a number on the fluctuation. Suppressed values tend to accompany accumulated stress and higher values a recovered state, though the relationship is far noisier within one person than group averages make it look ¹ ².

Higher is not reliably better either. In elite endurance athletes HRV can plateau or move in the opposite direction to what you would expect while adaptation is going well, and the same absolute value means different things in two different athletes ¹.

Conditions matter more than decimal places. Readings taken at different times of day, in different postures, or after caffeine are not comparable with each other, so taking it the same way every morning is what makes the series readable at all ².

One morning reading tells you almost nothing

Day-to-day scatter in a healthy, well-trained athlete is large enough to swamp the change you are trying to detect. The signal is the rolling average across about a week, plus how tightly the individual days cluster around it. A case comparison of two elite triathletes through an Ironman build found the interpretable information sat in the weekly mean and in the variation around that mean, not in any single day ³.

What is worth acting on is a mean that drifts down across one or two weeks, or a spread that widens while the mean holds. Both are visible only once you have a baseline to compare against ³ ¹.

Timing hard days by the trend

The original test of this gave one group a fixed programme and the other hard sessions only on days when HRV sat at or above their own baseline. The guided group improved more ⁴. The finding has since been repeated in trained runners ⁵ ⁶ and in trained cyclists ⁷.

The mechanism is placement, not accumulation. Guided groups in these trials did not do more hard work than their fixed-plan counterparts. In some cases they did fewer hard sessions and still came out ahead, because the sessions landed on days the athlete could absorb them ⁷ ⁶. Readiness also works upward: a trend sitting above baseline is a reason to take the hard session now rather than wait ⁵.

The evidence base is real but thin: small groups, study periods of a few weeks to a few months, and modest effect sizes. Treat guided timing as a better default than a fixed calendar rather than a large edge ⁴ ⁷.

Sleep

Short sleep costs endurance more consistently than it costs maximal strength, and it raises perceived effort at a workload you have handled before ⁸. That combination is why a debt week shows up as sessions feeling heavier at unchanged power before it shows up anywhere else.

The scale runs in both directions from your normal. College basketball players who spent several weeks aiming for ten hours in bed each night got faster sprint times, better shooting accuracy, and improved reaction time and mood ⁹. That was an extension above habitual sleep, not the repayment of a deficit.

What you report about yourself

A systematic review comparing the two families of measure found subjective self-report responded to changes in training load with greater sensitivity and greater consistency than the objective markers it was set against, across both acute and chronic load ¹⁰. Self-report is not the fallback for athletes without a chest strap.

Short daily scales covering fatigue, sleep quality, muscle soreness, stress and mood carry most of that information, and using the same scale every day matters more than which scale you picked. The recovery consensus frames the target as the balance between accumulated stress and accumulated recovery, which includes the stress that has nothing to do with training ¹¹.

Resting heart rate: cheap, stable, and slow to move, so it is useful mainly as corroboration ².
HRV trend: the seven-day mean and its spread, never yesterday alone ³.
Self-report: the most responsive single input, and the one that catches non-training stress ¹⁰ ¹¹.
Heart rate drift against power within a long steady effort: rises as the session wears on, and the size of the rise reflects fatigue rather than fitness on that day ¹².

References

Numbered in order of first appearance. Every link goes to the original work.