How to Read Your HRV, Sleep Score and Fitness Tracker Data

To read your fitness tracker data, compare your HRV and resting heart rate only with your own baseline, watch the 7-day average instead of single days, treat deep and REM sleep minutes as rough estimates, and ignore calorie burn. Your tracker is one input to how hard you train today, never the whole decision.

I love data. I also love watching women hand their entire mood for the day over to a little number on their wrist. (I don't actually love that. I'm being sarcastic.) Let's fix it.

What is a good HRV for my age?

There isn't one you should be chasing. HRV, heart rate variability, is the beat-to-beat variation in your heart rate. Most trackers report a measure called RMSSD, which mainly reflects the "rest and recover" side of your nervous system.

HRV is highly individual, and it falls with age. By your 50s it drops to about 47% of young-adult values, then levels off. And women's lower HRV compared with men's in youth disappears after 50.

So comparing your number with your friend's, or with an online chart, tells you NOTHING. Your friend's HRV is double yours? Great for her. Irrelevant for you. Compare yourself with yourself.

Should I skip training if my HRV dropped?

One low reading is one day. Here's how to read it properly:

• Use the weekly average. A 7-day rolling average picks up real change better than single readings, and it needs at least 3 readings that week. In runners, weekly-averaged HRV tracked fitness gains strongly, while single-day values barely did.
• Know your normal range. A change only matters if it moves outside your usual range, which sports guidelines define using your 60-day average and your normal day-to-day swing.
• Ignore scary labels. Device labels like "excessively fatigued" have little scientific support and shouldn't override how you feel.

What shifts HRV and sleeping heart rate: training load, sleep, nutrition, life stress, the menstrual cycle and alcohol. In 4,098 adults, even LOW alcohol intake raised sleeping heart rate by about 1.4 beats and lowered HRV. High intake raised it about 8.7 beats and dropped the recovery score by about 39 points. (I don't drink. My HRV thanks me.)

If your weekly average keeps sliding along with poor sleep or a higher resting heart rate, that's the pattern worth acting on. Want to nudge it in the right direction? Start with [the 5-minute breathwork practice](/blog/5-minute-breathwork-practice-hrv-calm).

What does a higher resting heart rate mean?

A resting heart rate clearly above your own baseline for a few days is a useful flag for illness, poor sleep, alcohol or stress. In one smartwatch study, 26 of 32 COVID-19 cases showed changes in resting heart rate, steps or sleep, often at or before symptoms started.

Up 6 beats for three days? Worth noticing. Prioritise sleep, ease off if you feel off, and if you feel unwell, rest and check with your doctor. Like HRV, judge it against your own trend, not one morning.

How accurate are sleep trackers for deep sleep?

Not very, for stages. Compared with lab sleep studies, consumer trackers are very good at detecting that you were asleep but poor at spotting time you lay awake in bed. Stage estimates (light, deep, REM) were mixed, devices often overestimated light sleep, and accuracy dropped on disturbed nights.

Which means your deep and REM minutes are least reliable on exactly the nights you worry about. Night sweats. 3 am waking. The nights you check your score first thing and decide you're "wrecked".

And chasing a perfect sleep score can itself make sleep worse. Researchers have a name for it: "orthosomnia". So judge the night by how you feel, not one sleep stage on your tracker. Your subconscious believes what you tell it in the morning. "I slept terribly, the app says so" is a story you just installed for the whole day.

Can I eat back the calories my tracker says I burned?

No. Wrist heart rate is reasonably accurate, with a median error of about 2 to 7% in a lab study. Calorie burn errors ranged from about 27% to 93%.

Ninety-three percent. That's not a number, that's a horoscope. Heart rate and trends are the useful parts. Calorie burn is the least accurate number on any tracker, so your calorie targets stay the ones your plan sets. For how to actually set them, see [macros 101](/blog/macros-101-high-protein-performance).

Readiness, strain and training load scores come from each maker's own unpublished formulas built on heart rate, HRV, sleep and movement. Use them as a trend and a conversation starter, not a verdict. A red readiness score is a prompt, not an order.

Why did my body scan jump overnight?

Because most home body composition scans use bioimpedance: a small current estimates fat and lean mass from your body water. It works reasonably well in healthy people with stable hydration, but not being fasted, scanning in the afternoon or scanning after a meal all produced big shifts in fat and lean mass estimates.

And creatine raises total body water, so your scan may show a lean mass or water shift soon after you start it. That's water, not fat.

How to scan:

• First thing in the morning, after the bathroom
• Before food, drinks, coffee or exercise
• Same device, same conditions, every time
• Trust the two-week average, never one scan

How should tracker data change my training?

It shouldn't change your program. It should inform how hard you push today. In PRYME that's the Daily Check plus Today's Call: go hard or ease off. Tracker data is one input. How you feel, logged honestly, counts just as much. On days you feel off for any reason, including cycle or perimenopause symptoms, lean on that readiness check, not a calendar.

This isn't medical advice, and your tracker can't diagnose anything. Irregular heart rhythm alerts, chest pain, fainting, blood oxygen alerts, or a resting heart rate that stays unusually high or low with symptoms: see your doctor. And if scan numbers are pulling you to cut food or add exercise to "fix" them, talk to a coach.

Quick answers

What's a normal HRV for a woman over 50? HRV is highly individual and falls with age, so there isn't a target number. Compare your 7-day average with your own baseline, not a chart.

My HRV dropped today. Should I skip my workout? One low reading is one day. Check in with how you feel, and act on the pattern if your weekly average keeps sliding with poor sleep or a higher resting heart rate.

Are sleep trackers accurate for deep sleep? They're good at knowing you were asleep but rough at sorting light, deep and REM sleep, especially on disturbed nights.

How accurate is tracker calorie burn? Not very. Errors ranged from about 27% to 93% in a lab study, while heart rate was within about 2 to 7%.

Why did my body fat go up after starting creatine? Creatine raises total body water, which shifts bioimpedance scan readings. That's water, not fat.

Sources. Bellenger C, et al. Best practice guidelines: measurement, analysis and interpretation of resting heart rate and heart rate variability in athletes. Australian Institute of Sport. 2023. Plews DJ, et al. Training adaptation and heart rate variability in elite endurance athletes. Sports Medicine. 2013. Plews DJ. The practical application of heart rate variability: monitoring training adaptation in world class athletes. Doctoral thesis, Auckland University of Technology. 2014. Umetani K, et al. Twenty-four hour time domain heart rate variability and heart rate: relations to age and gender over nine decades. Journal of the American College of Cardiology. 1998. Pietilä J, et al. Acute effect of alcohol intake on cardiovascular autonomic regulation during the first hours of sleep. JMIR Mental Health. 2018. Mishra T, et al. Pre-symptomatic detection of COVID-19 from smartwatch data. Nature Biomedical Engineering. 2020. Chinoy ED, et al. Performance of seven consumer sleep-tracking devices compared with polysomnography. Sleep. 2021. Baron KG, et al. Orthosomnia: are some patients taking the quantified self too far? Journal of Clinical Sleep Medicine. 2017. Shcherbina A, et al. Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. Journal of Personalized Medicine. 2017. Kyle UG, et al. Bioelectrical impedance analysis, part II: utilization in clinical practice. Clinical Nutrition. 2004. Kerr A, et al. Impact of food and fluid intake on technical and biological measurement error in body composition assessment methods in athletes. British Journal of Nutrition. 2017. Powers ME, et al. Creatine supplementation increases total body water without altering fluid distribution. Journal of Athletic Training. 2003.