The Runner's HRV: Training with Data

In 2007, at the University of Jyväskylä in Finland, exercise physiologist Ari Kiviniemi designed a fascinating experiment. He divided middle-aged male athletes into two groups. One group followed the traditional approach—a fixed training plan. "Monday is intervals, Wednesday is tempo run, Friday is long run."

The other group was different. They measured HRV every morning and adjusted training intensity based on the results. High HRV meant train hard; low HRV meant go easy.

After four weeks, the results came in.

The HRV-guided group showed greater improvement in cardiovascular fitness. And fewer people burned out from overtraining.

Seems obvious in hindsight. Train hard when your body is ready, rest when tired. But that's not how we usually train. "It's Wednesday, so intervals." We follow the calendar. Ignore how our body actually feels.

But your body doesn't check the calendar. Condition varies based on whether you worked late yesterday, drank the night before, or how much you trained last week. HRV shows that in numbers.

The 5:30 AM Dilemma

The alarm rings. Today's plan: 10km tempo run. But last night there was a work happy hour. A couple of beers, maybe a cocktail. Got home at 11 PM. Slept five and a half hours.

The internal debate begins while still under the covers.

Should I run or skip it?

Pushing through might make tomorrow even harder. But skipping feels like breaking training momentum. This same dilemma repeats every morning.

Here's an interesting data point. You open your health app and check HRV: 38ms. Your baseline is 48ms, so that's about a 20% drop. Sleep was 70% of normal. Honestly, your body feels heavy too.

This situation is a clear red light.

The smart choice? Skip the tempo run. Replace it with very easy jogging for 3-4km, or take a complete rest day. Rest well today, and tomorrow you'll be recovered. Do the tempo then. Just one day delayed.

Your Heart Doesn't Beat Like a Machine

Let's briefly review what HRV is.

It's easy to think your heart beats exactly once per second—tick, tick, tick. It doesn't. The intervals constantly change: 0.8 seconds, 1.1 seconds, 0.9 seconds... Measuring this variation is HRV (Heart Rate Variability).

The fascinating part: more variation is better.

Think of a car. A good car can flexibly adjust speed for road conditions. Our bodies work the same way. Being able to flexibly adjust heartbeat means being ready to respond to various situations.

Low HRV? Your heart is beating nearly uniformly. A signal that your body is already busy dealing with something. Maybe sleep deprivation, training fatigue, or work stress.

The key insight: HRV shows numerically how much reserve capacity your body has right now.

Why Morning?

HRV changes throughout the day. Coffee changes it. Climbing stairs changes it. Lunch changes it. A stressful meeting changes it.

That's why you need to measure under consistent conditions. Otherwise you can't compare.

Morning is optimal for a reason. You've slept 6-8 hours, haven't had caffeine or food yet, haven't done physical activity, and mental stress is minimized.

Good news: this is now automated. Sleep wearing an Apple Watch and it records sleep HRV automatically. Same with Oura Ring or Garmin. By morning, data is already waiting.

Just a few years ago, you needed to strap on a heart rate belt and lie still for 5 minutes. Now you just sleep.

How to Read the Numbers

"45ms today. Is that good?"

The question itself is wrong.

HRV absolute values vary wildly between people. A 20-year-old athlete might average 60ms; a 40-year-old office worker might average 35ms. Both are normal.

What matters is how today compares to your personal average.

Use your last 3 weeks (21 days) average as your baseline. Then see what percentage today deviates from that baseline.

If your baseline is 50ms and today is 45ms? That's a 10% drop. This percentage is the key metric.

Why 21 days (3 weeks)? Too short and you're swayed by temporary fluctuations. Too long and you miss fitness changes. Three weeks is the sweet spot.

The Traffic Light System

How to synthesize all information and decide today's training. The simplest, most effective approach is the traffic light system.

Green Light: GO as Planned

HRV within 3% of baseline
Sleep at 90%+ of normal
One or fewer high-intensity sessions in the last 3 days

On these days, push according to plan. Tempo run planned? Do the tempo. Intervals planned? Do intervals. Resting too much on days like this wastes good condition.

Yellow Light: CAUTION - Adjust Intensity

If any of these apply, it's a yellow light:

HRV 3-10% below baseline
Sleep only 6-7 hours (insufficient)
Two consecutive recent high-intensity sessions

Yellow doesn't mean stop. It means adjust intensity. Easy run instead of tempo. Drills instead of intervals. Keep moving without overdoing it.

Red Light: STOP - Recovery First

If any of these apply, it's a red light:

HRV 10%+ below baseline for two consecutive days
Sleep under 80% of normal AND body feels heavy

Push through a red light? Short-term, you might manage. But accumulate days like this and problems arise. Chronic fatigue. Performance decline. Eventually, injury.

It's far better to take a light day or two than to spend two weeks injured during marathon prep.

The Opposite Situation Exists Too

Saturday morning. Long run planned: 20km. Last night you went to bed early and got 8 hours of solid sleep. Check HRV: 52ms. Higher than your 48ms baseline. Body feels light too.

Perfect green light.

Run the planned 20km. Resting too much on days like this means wasting good condition.

A 2021 study on cyclists showed similar results. HRV-guided training was as effective as traditional periodization—with the added advantage of individual customization.

Think about it. We typically train by calendar: intervals on Monday, tempo on Wednesday. But your body doesn't check the calendar.

As Data Accumulates, Patterns Emerge

Track HRV for a month, two months, a year, and interesting patterns appear.

Alcohol and HRV. Next-day HRV drop after drinking is almost universal. Even two or three beers have an impact. Heavy drinking affects you for two or three days. Alcohol impacts the autonomic nervous system—the sympathetic system stays activated even during sleep.

Interestingly, knowing this pattern often leads to self-regulation. "Important training tomorrow, so just one drink tonight." Numbers change behavior.

Cumulative Sleep Debt. One night of short sleep is fine. Good sleep the next night recovers it. But two, three consecutive nights are different. Day one seems okay, day two starts dropping, day three falls sharply. And HRV depleted this way doesn't bounce back with just one good night's sleep. It takes several days.

"I'll cut sleep on weekdays and catch up on weekends"—HRV data shows it's not that simple.

The Rhythm of Training and Recovery. High-intensity training drops HRV. Normal. Your body is responding to exercise stress. Rest adequately and it rises again. This ebb and flow is the training adaptation process.

The problem is when HRV stays down and won't come back up. It may signal training load exceeding recovery capacity. Time to modify your plan and secure more recovery time.

Conversely, if HRV is so stable there's barely any change? May signal insufficient training stimulus. Of course, during race taper, stability is good. But during base-building, some variation is normal.

Work Stress Matters Too. Mental stress shows up in HRV. Before a big presentation or during difficult team situations, HRV naturally drops.

During such times, training itself can help relieve stress. But adjust intensity. Easy-pace jogging beats high-intensity intervals. Run to clear your head, shower afterward, and stress often dissolves.

The key is ensuring training doesn't become another stressor.

Frequently Asked Questions

"HRV is low but I feel great."

This happens often. Especially the day after high-intensity training. After hard workouts, sympathetic activation can persist 24-48 hours. So morning HRV is depressed, but fatigue hasn't hit yet.

On these days, light exercise is fine. Just avoid high intensity. In a day or two, HRV and condition will align.

"Opposite situation: HRV is high but I'm tired."

This is possible too. Several reasons exist. Could be measurement error—loose wristband during sleep reduces accuracy. Or maybe you did very light, long exercise yesterday, or meditated, activating the parasympathetic and raising HRV. But this might not reflect actual recovery.

That's why you can't look at HRV alone. Consider sleep duration, recent training load, and subjective feel together.

"I have a race coming up?"

During taper, HRV should gradually rise. You're reducing training volume, so your body recovers. But if HRV keeps dropping or won't rise despite tapering? You need to cut volume more aggressively. The goal is peak condition on race day.

High HRV the night before or morning of race day is a good sign. Your body is ready. Even if nerves or poor sleep drop HRV, don't worry too much. Race-day adrenaline compensates for a lot.

"Different devices show different numbers?"

Normal. Apple Watch might show 50ms while another app shows 30ms. Multiple HRV calculation methods exist. Apple Health typically shows SDNN; Oura Ring mainly shows RMSSD. Different calculations produce different absolute values.

Comparing absolute values between devices is meaningless. What matters is your trend from the same device. If you chose Apple Watch, keep looking only at Apple Watch. Not switching devices is key to consistent data.

Start Tomorrow Morning

No need to aim for perfection from the start.

First 3 days: just observe. If using Apple Watch, add Heart Rate Variability to favorites in the Health app. Starting tomorrow morning, check right after waking. Keep notes like this:

11/25: HRV 48ms, 7 hours sleep, easy jog yesterday
11/26: HRV 52ms, 7.5 hours sleep, rest day yesterday
11/27: HRV 41ms, 5.5 hours sleep, work dinner yesterday

After a few days, patterns emerge. "Definitely drops after work dinners." "Goes up after good sleep."

After one week, run your first experiment. When HRV is 10%+ below baseline, reduce planned training intensity by 20-30%. Then check how you feel the next day. Experiences like this accumulate, and you'll feel how HRV actually reflects your body's state.

Try the opposite experiment too. Some days HRV will be above baseline and condition feels good. On those days, train slightly harder than usual. You'll probably run much better than normal.

Final Thoughts

For working runners, time is scarce. Wake early to run, go to work, spend evenings with family or friends, then back to bed. Just making time to train amid this busy life is an achievement.

That's exactly why you want to use that precious training time as effectively as possible.

Pushing until injury and missing weeks is far worse than adjusting intensity appropriately and running consistently. Whether half marathon or full, the one who crosses the finish line is the one who prepared consistently.

HRV can be a small compass on that journey.

You don't have to be ruled by numbers. HRV is reference material. You make the final decision. If numbers show green but your body feels off, rest. If numbers show yellow but you really want to run, go for an easy jog. The point is that having HRV as additional information helps you make wiser choices.

Injury-free, burnout-free, consistent. Isn't that what we really want?

Tomorrow morning, check when you wake up. What story is your body telling you today?

References

Kiviniemi, A.M., et al. (2007). Endurance training guided individually by daily heart rate variability measurements. European Journal of Applied Physiology.
Bellenger, C.R., et al. (2016). Monitoring Athletic Training Status Through Autonomic Heart Rate Regulation. Sports Medicine.
Javaloyes, A., et al. (2021). Training Prescription Guided by Heart Rate Variability vs. Block Periodization. Journal of Strength and Conditioning Research.
Plews, D.J., et al. (2013). Heart rate variability in elite triathletes, is variation in variability the key to effective training? European Journal of Applied Physiology.