TL;DR
- Resting heart rate (RHR) is a simple signal of stress and fatigue. It often rises with poor sleep, illness, dehydration, and heavy training.
- HRV (usually SDNN on Apple Health) is a signal of autonomic balance. It is best used as a trend, not a single reading.
- When HRV and RHR disagree, it usually means you have a mixed signal: you may be tired but adapting well, or stressed in a way that is not purely training load.
- Use both metrics with context: sleep quality, soreness, and recent training matter.
- Century combines HRV, sleep, and training load to make the signals easier to act on.
Why people compare HRV and resting heart rate
If you wear an Apple Watch (or any wearable), you will eventually see this pattern:
- your resting heart rate goes up
- your HRV goes down
It feels like they are opposites.
Often they are.
But they are not interchangeable, and the situations where they disagree are where you can make the best decisions.
What resting heart rate is actually telling you
Resting heart rate is the lowest stable heart rate you hit during rest, usually during sleep.
In practice, RHR is influenced by:
- sleep duration and timing
- dehydration
- alcohol
- illness (even before symptoms)
- accumulated training fatigue
- heat
- psychological stress
RHR is powerful because it is simple. If your baseline is stable and it suddenly jumps, something is up.
The downside of RHR
RHR is not very specific.
A higher RHR could mean:
- you trained hard and need an easier day
- you are fighting a cold
- you slept badly
- you drank alcohol
- you are dehydrated
Same signal, many causes.
What HRV is actually telling you
Heart rate variability is a measure of how much the time between heartbeats varies.
In Apple Health, HRV is typically reported as SDNN.
HRV tends to be higher when you are:
- well rested
- adapting well
- in a calmer parasympathetic state
It tends to be lower when you are:
- under stress
- sleep deprived
- sick
- overreaching
The downside of HRV
HRV is sensitive.
It can swing due to:
- measurement timing (morning vs night)
- motion artifacts
- irregular sleep
- a single stressful day
That is why HRV is best used as a trend with a baseline, not as a daily grade.
HRV vs RHR: the practical differences
Think of it like this:
- RHR answers: "How hard is your body working at rest right now?"
- HRV answers: "What is my nervous system balance and recovery capacity trend?"
They often correlate, but they represent different layers.
What to do when both signals are "bad"
Scenario:
- RHR is above baseline
- HRV is below baseline
This is the classic "take it easy" day.
Best moves:
- do a low intensity session (easy run, Zone 2)
- shorten volume
- prioritize sleep and hydration
- avoid stacking intensity
If you also have symptoms (sore throat, fever, aches), skip training.
What to do when both signals are "good"
Scenario:
- RHR is at or below baseline
- HRV is at or above baseline
This is a good day for:
- quality training (intervals, tempo)
- longer sessions
- progressive overload
Still: do not ignore soreness, sleep, or life stress.
The interesting part: when HRV and RHR disagree
Case 1: HRV down, RHR normal
Common causes:
- psychological stress
- irregular sleep timing
- travel
- late meals
What it can mean:
- you might feel okay, but your nervous system is not fully recovered
What to do:
- keep training, but reduce intensity or volume slightly
- focus on a strong warm up
- if you feel flat, pivot to an easy session
Case 2: RHR up, HRV normal
Common causes:
- heat exposure
- dehydration
- alcohol
- a hard session the day before
What it can mean:
- you may be physiologically strained, even if HRV has not dropped yet
What to do:
- hydrate and fuel well
- keep intensity controlled
- prefer endurance over all out intervals
A simple decision framework you can use today
Use this checklist before you decide how hard to train.
- Sleep
- did you sleep enough?
- did you wake up a lot?
- Body
- any illness symptoms?
- soreness and heaviness?
- Wearable signals
- RHR vs baseline
- HRV trend vs baseline
- Training plan
- is today a key session?
- can you move it 24 hours?
If multiple items are off, it is rarely worth forcing intensity.
Two good videos on recovery signals
Where Century fits
Most people do not need more metrics. They need clearer decisions.
Century helps by:
- building your baseline for HRV and resting heart rate
- connecting those trends to your sleep and training load
- highlighting when a "weird" day is likely travel, stress, or training fatigue
If you want recovery guidance that adapts to your week, Century is built for that.