HRV, resting heart rate and the Nervous System

What this page covers#

On the Overview, the row beside the energy map holds two cards of your choice. The name in a card's title opens a menu, and "Overnight HRV" is one of the six choices. The cards on these tabs are Premium, see Free and Premium.

Sleep, deep sleep and the sleep score are explained in sleep quality, Recovery in Recovery and Training Readiness, and the heart-rate zones in heart-rate zones. If you want the plain definition of HRV first, the health glossary has it.

Overnight HRV#

What your device delivers#

Most wearables report one number per night: the average heart rate variability during sleep, in milliseconds. The measure is called RMSSD (the typical size of the change between neighbouring heartbeats), and the app labels it "HRV (RMSSD)". We accept a night between 5 and 250 ms and ignore anything outside, as a sensor error. One night is kept per date.

Some devices also report the best five minutes of the night (used by Nervous System, see below) and a usual range of their own (Garmin does). The device's own weekly figure is not used for your week: we compute the week from your nights.

Different devices measure HRV on different scales, so 40 ms from one is not 40 ms from another, and nobody's number is compared with yours. The only comparison that carries information is you against your own history. For the same reason, if two of your devices both deliver HRV, see two devices.

Only the overnight HRV is scored. A reading taken after waking, such as a watch's two-minute morning measurement, is not used in any score.

The week, not the night#

A single night is noisy: a late meal, alcohol or a poor position of the sensor can move it by a lot, and in research the weekly mean tracks changes in running performance far better than one day does (Plews and colleagues, 2013). So the number we score is the week.

1. For each night we take the natural logarithm of the HRV. HRV is not spread evenly, and the logarithm makes averaging fair.
2. We average the nights that fall in the last 7 calendar days, counting the day being scored.
3. We convert the average back to milliseconds. This is the "7-night average" the card shows as the large number, labeled "mean of 5 nights this week".

The week needs at least 3 nights. With fewer, the card shows a counter such as "2/3 nights this week" and says "2 of 3 nights this week, the mean shows from the third". Three nights is the least a week can rest on in the studies we follow (Plews and colleagues, 2014). Days count, not rows: a night from nine days ago is not in the week, however few nights there are.

The ladder and your own shift#

To turn the weekly milliseconds into a 0 to 100 score we use two steps.

Step 1, a fixed ladder. It is our own curve, not a population norm: 100 / (1 + e^(-0.06 × (ms - 40))).

Step 2, a shift from your own history. We take where your week sits among your own weekly averages of the last 60 days (see your baseline). At your own mean nothing moves. At two standard deviations away (a standard deviation is how widely your numbers usually scatter) the score moves by 20 points, up or down, and never further. Until you have at least 7 weekly averages the shift is skipped and the ladder alone stands.

The shift exists because the same milliseconds mean different things on different devices: a Fitbit wearer and a Garmin wearer in the same shape sit tens of milliseconds apart.

Your baseline#

The baseline is built from your nights of the last 60 days:

• for every day that has a night, we compute the 7-night average ending on that day, but only when the week holds at least 3 nights;
• the baseline is the mean of these rolling weekly averages, and its spread is their standard deviation, how widely they scatter;
• it exists once there are 7 such weekly averages.

Nine nights in a row are enough to give you 7 weekly averages (they overlap), so the shift above can start early. The band, which is a stricter promise, waits longer.

Your band#

The band is the range inside which a week counts as "your usual". Its edges are half a standard deviation either side of the baseline, where the deviation is that of your single nights (on the logarithmic scale). Half a standard deviation is the smallest change that research treats as worth noticing (Plews and colleagues, 2012; Buchheit, 2014).

The band appears only when your nights span at least 28 days from the first to the last in the 60-day window, and the baseline exists. Until then the card says "Learning your band" and counts down: "12 more days at the earliest", or "It needs more nights" when the span is long enough but the nights are too sparse.

Where the week sits against the band is written as one of these, with the rule behind it:

The score and the band use two different spreads on purpose. The score uses how much your weekly averages vary, which is the right ruler for a week. The band uses how much single nights vary, which is the published smallest worthwhile change, and is about twice as wide.

Last night, and how it fades#

The newest night is scored on the same ladder and shifted by where it sits among your single nights. It then counts by how old it is, in days from the scored day:

Recovery uses it as a smaller term next to the week (Recovery). The week is never replaced by it: a very bad night is real news on the morning it happens, but it is one night. Nervous System uses the newest night in place of the week only when the week has fewer than 3 nights and the newest night is less than 7 days old.

In the example above, last night of 41 ms scores on the ladder at 51 and is shifted by ln(41 / 50) / 0.20 = -0.99 single-night spreads, that is -9.9 points: 41.

Two devices, the phone, mornings#

• One device at a time. If your nights come from two devices, the scores read the nights of the device that wrote your newest night, each device against its own band. A device on another scale never joins your window, so changing devices does not look like a collapse of your HRV. If the second device's own week sits on the same side of its own band, the card adds "[Device] agrees: its week is below its own band too", otherwise "[Device] does not confirm it ... It may be one device's swing; watch whether the next days confirm it." We never print the second device's milliseconds beside yours.
• The phone yields to a wearable. Overnight HRV that the phone averages from Health Connect is on its own scale. While a wearable has written a night within 30 days before your newest night, the phone's nights leave the window. Past that, you have moved to the phone, and the wearable's old nights leave instead.
• Garmin's own band. A Garmin wearer sees a band in Garmin Connect too. Garmin does not describe how it draws it, and ours comes from your nights over the last 60 days, so the two can differ. The card says so.

The card and what it says#

• Large number: the week in ms, in the color of its state, with "mean of 5 nights this week" beside it.
• Verdict box: one sentence, for example "Your nights are slightly below your band this week.", then a line on what it means ("It is just past your usual variation." or "It is more than your usual variation."), and, for a low week on a card without the coach's plan line, the advice "An easier day makes sense; what counts is whether it holds over the next days." A week above the band reads "Above the band means more than usual; on its own it is not a warning."
• Resting heart rate line: see below. A resting rate that is up beside an HRV under its band deepens the color one step.
• Rail: a ruler with a pin for the week against the band, and "weekly variation 4.2 %", which says how widely the week's nights scatter.
• Charts: "14 days" (the week on screen beside the week before it), "30 days", and "HRV and HR", a small map that trails the week ending 12, 8, 4 and 0 days back against your resting heart rate.
• Plan line on the Health tab: "Today's plan: easy", the coach's verb, never advice of the card's own. In the Overview pair the plan is left to the coach.
• Empty states: "Too few nights" when the week has under 3 nights but the band exists, "Learning your band" while the band is being built. On a Garmin account with no night in the last 60 days the card stays, empty, with the way to get the data. On any other account the card disappears when the newest night is more than 14 days old.

What the band does to today's plan#

A week below the band caps the coach's verb for the day at EASY, however green the rest looks. If the week has been below the band for at least 14 days in a row, with no hard session yesterday and a weekly load no higher than your month, the cap becomes MODERATE instead, because easy days answer training fatigue and cannot answer poor sleep, stress or an infection; the banner then says so. A week within or above the band changes nothing here. See the daily coach. Body Status also reads the band: a week below it vetoes a Peak Day (Body Status, Peak Day).

What moves your HRV#

Under the card sits a folded panel, "What moves your overnight HRV". For each factor it compares your nights after the factor with your other nights over the last 90 days, and prints the difference in per cent.

The difference is the average of the logarithms of the first group against the second, converted to per cent. A figure appears only with at least 3 nights with the factor and 5 without it. It is called "clearly" when it is big against the nights' own scatter (a t value of 2 or more) and at least 3 %, "a hint" from a t value of 1, and "within the noise" below that. A clear drop is drawn in warm color. Alcohol and illness compare only against nights that have a check-in, because a night with no check-in is not a night without a drink. The panel shows when you have at least 8 nights in the window, and the folded line shows the strongest clear difference ("Most: After alcohol -12 %").

It reports what came before a low or high night, not what caused it: the words say "after", never "because of".

Resting heart rate#

How each device gets it#

The resting heart rate is the pulse at rest. Devices define it differently, and we use the device's own figure where it states one:

• Oura: the lowest of the five-minute averages of the night's main sleep.
• Garmin (direct link and export): Garmin's own daily figure, the lowest 30-minute average of the day, as Garmin Connect shows it.
• Fitbit and Google Health, COROS: the device's own daily resting rate.
• Polar: we work it out from the heart-rate samples inside your sleep window, as the average of the lowest tenth of them. The same is done for an Oura night for which the ring filed no figure. It needs at least 15 samples in the night and at least 3 in that lowest tenth, and it needs a sleep window, because we will not guess when you slept.

A pulse taken by a scale or a blood-pressure cuff, standing or sitting, is not a resting heart rate and is kept apart. The lowest pulse of the night (the "lowest heart rate at night") is a separate reading, used on its own.

The card line and the week#

Inside the HRV card the line "Resting heart rate" prints this week's average against your last two months: "1.6 bpm above your usual (56.2)", or "as usual". The week needs at least 4 days with a resting rate and the two months at least 20. A difference of 1 bpm or more counts as a move. When it is up, the card adds that resting heart rate normally varies by about 3 bpm from day to day, so the whole week is compared, and that it says more when it stays up for several days.

What reads your resting heart rate#

The resting rate has no score of its own. It goes into Nervous System, Cardiac Fitness (twice, as a level and as steadiness), Body Alert (the rise against the days before, a floor for a high level, and a jump of more than 5 bpm from yesterday, see Body Alert), Immunity (a deduction), Overtraining, the heart-rate zones as the base of your reserve (zones) and biological age. Recovery reads the lowest pulse of the night instead of the resting rate. Safety messages about a resting rate over 100 or under 40 are in Body Alert.

For women, 5 bpm is taken off the pulse before it is placed on a ladder in Nervous System and Cardiac Fitness, because resting pulse runs about 5 bpm higher in women.

Nervous System#

Nervous System (Czech "Nervový systém") is a 0 to 100 index, higher is better. Despite the name, it is not a balance between the two branches of the autonomic nervous system: that needs a frequency analysis of HRV that a wrist device does not do. It is a recovery tone: every part is on the recovery side (variability, a low night pulse, low stress, morning charge). The card says so: "the recovery branch of your autonomic nervous system, read from your HRV".

The seven parts#

A part you do not have drops out and the rest are scaled up to 100 %.

• HRV, 7-night average. As in the ladder. In a week with fewer than 3 nights, the newest night stands in (if it is less than 7 days old), and the sheet then calls it "HRV, last night".
• HRV, best 5 minutes. On its own curve centered on 60 ms: 40 ms scores 27, 60 scores 50, 80 scores 73, 100 scores 88. It is then shifted by your own peaks, once you have enough of them.
• HRV against the device's own range. At or above the device's high edge, 95. Between the middle and the high edge, from 70 to 95. Between the low edge and the middle, from 45 to 70. Under the low edge, 45 minus twice the gap, never below 10. It is not drawn on the sheet, but it is in the average.
• Stress. Only the 0 to 100 index (Garmin, and COROS on the same scale) is used here. Minutes of stress from an Oura ring are not, because they have no published scale (stress). Today is left out, because a day still running would move the number through the afternoon. The week's average goes on the stress ladder, and the newest completed day shifts it by up to 20 points.
• Resting heart rate. The week's average on the line 160 minus 1.7 times the rate (50 bpm gives 75, 60 gives 58, 70 gives 41), then shifted by up to 20 points by the newest reading against your own 30-day usual (a rate above your usual lowers the score).
• Lowest heart rate at night. 140 minus 1.8 times the rate (45 bpm gives 59, 50 gives 50, 60 gives 32). No personal shift.
• Body Battery at waking. The device's level at waking, averaged over the last 5 days, taken as it is (0 to 100). It is Garmin's Body Battery, never our Morning Energy estimate.

After the average#

1. The trend nudge. From your last 7 nights we fit a line through the logarithm of the HRV, read as milliseconds per day at a reference of 40 ms. A rise of 1.5 ms a day or more adds 6 points. A fall of 3.0 ms a day or more takes 5 points. Smaller moves change nothing.
2. Thin data. With only 2 parts the score keeps 60 % of its distance from 50, with 3 parts 80 %, with 4 or more all of it. A score of 80 on two parts is shown as 68.
3. The number is rounded to a whole number.

The index needs at least one part that says something about variability (HRV, the best five minutes, the device's own range, stress or morning Body Battery). A resting rate and a lowest night rate alone are two heart-rate levels and never make a Nervous System. It also needs overnight HRV or a resting rate on at least 3 days, and a reading inside the last 7 days: with none, the card says "No recent data" and shows when it was last measured.

The "Built from" list in the metric sheet prints each part with its value, your usual and its share (for example "HRV, 7-night average 44 ms, usually 50, 27 %").

Cardiac Fitness#

Cardiac Fitness (Czech "Srdeční kondice") is a 0 to 100 index of how aerobically fit the heart looks from your resting pulse and HRV. It is not the "cardiac efficiency" a doctor measures with an ultrasound; the card says so.

• Resting heart rate. 50 bpm gives 65, 60 gives 50, 70 gives 35. The newest reading against your own 30-day usual shifts it, by up to 12 points here (not 20), because the resting rate is read twice in this index and one unusual morning should not decide the heart.
• HRV against pulse. The ratio of average overnight HRV to average resting rate, on the curve 100 / (1 + e^(-4 × (ratio - 0.7))): a ratio of 0.3 gives 17, 0.5 gives 31, 0.7 gives 50, 1.0 gives 77, 1.5 gives 96. No personal shift.
• Steadiness. 100 minus 12 times the standard deviation of the week's resting rates, so a week that varies by 2 bpm gives 76. It needs at least 5 resting rates in the week.
• Lowest heart rate at night. 50 bpm gives 50, 45 gives 58, 60 gives 34.

For women, 5 bpm is taken off the resting rate and the lowest night rate first. The parts are averaged over those present, and with 2 or 3 parts the score is pulled toward 50 as above.

Minimum data. At least 5 days of rows in the last 30, a resting rate on at least 7 days, a reading inside the last 7 days, and at least two independent measurements among resting rate, HRV and lowest night rate. A pulse a scale takes while you stand on it, on its own, never makes a heart score. Without them the index is withheld, not guessed.

Vascular age#

Vascular age is not a score we compute. It is a number your device delivers, which we show on the "Vascular age" card in the "Vascular & metabolic" section of the Health tab, next to your calendar age. A Withings scale (Body Cardio, Body Scan) measures the speed of the pulse wave through your arteries, and the years are derived from that velocity. An Oura ring estimates the pulse wave from the optical signal of the ring and delivers a number after about two weeks of wear. The card says which of the two stands behind your number and never mixes them.

The card compares the value with your age, using the birth date in Settings: 1 year or more younger shows green, from less than 1 year younger up to 5 years older is amber and described as "within ±5y tolerance", and more than 5 years older is red. It is "An indication, not a medical diagnosis." When the scale's measured velocity and age stand together, the age feeds the cardiovascular part of biological age. Everything about that is in biological age.

Age and sex in all of this#

• HRV is not adjusted for sex or age in Recovery or Nervous System. The personal shift takes care of what a body's own level is.
• Resting rate and lowest night rate carry the 5 bpm offset for women in Nervous System, Cardiac Fitness and Body Alert; Body Alert also takes 0.15 bpm off the rate for each year of age past 30.
• A reference by age and sex is used in one place, the floor under Body Alert for a persistently low HRV. It comes from a study of overnight RMSSD in millions of people wearing a wrist sensor (Natarajan and colleagues, 2019 preprint). Past age 60 the values continue on the study's power law of RMSSD against age. If your age is not known, we assume 40.

When it updates#

• The week, band, scores and the card are worked out when your night arrives, and again whenever new data arrive. They do not drift through the day, except that today's stress is left out until the day has ended.
• A night that has not arrived yet is not guessed from nothing: for Body Status the day is scored on your usual night and marked "Provisional" (Body Status). The HRV card simply shows the last night on record, with its date.
• Results are reused for up to 5 minutes, and a sync refreshes them. A past day, opened in the day picker, is recalculated for that day, with its own week and band at that date. When we change how a number is calculated, past days are scored again.
• "Fetch new data" pulls from your device (sync). Garmin nights come through the Garmin link or an uploaded export, because Garmin puts no overnight HRV into Health Connect; other brands can reach us through the phone app (devices).

Free and Premium#

On the free plan (from 1 November 2026) these cards, the Health, Sleep and Trends tabs and past days are Premium. Free keeps today's Body Status and the safety alerts. Your data keeps arriving and nothing is lost. Premium includes the 7-day trial (plans).

Energy pacing mode#

The calculation does not change. In the mode, the plan line of the HRV card is replaced by the pacing banner when the week is held under the band it had before a drop, and "stress" is called "body load" in the Nervous System sheet and elsewhere (energy pacing).

What you can do#

• Wear the device to bed, regularly. The week needs 3 nights, the band 4 weeks.
• Compare yourself with yourself. Ignore other people's milliseconds.
• Read the week, not the morning. One low night after alcohol or a late session is ordinary.
• Look at "What moves your overnight HRV" before you worry. It often names the reason.
• Keep sleep, alcohol and evening training steady for a week if you want to see your own baseline.
• If your HRV week and resting heart rate both turn the wrong way and you feel unwell, rest, and see a doctor if it does not ease.

MyBodyAI is a wellness app, not a medical device. HRV, Nervous System and Cardiac Fitness describe signals from your device and do not diagnose. If your heart rhythm is irregular, HRV numbers do not mean what this page describes: ask your doctor.

Frequently asked questions#

Why is my HRV different from my watch's own app?#

Our week is the average of your last 7 nights on a logarithmic scale, and our band is built from your nights of the last 60 days. Your watch may show last night alone, or its own weekly figure, and may draw its own range by a method it does not describe. The nights are the same, so the direction should agree; the numbers and the range need not.

Why does it say "Learning your band"?#

The band needs nights that cover at least 28 days, from your first night in the last 60 to your newest, and a baseline of at least 7 weekly averages that each hold 3 nights or more. The card counts down the days it needs at the earliest. The Nervous System score starts earlier, because its personal shift needs only the 7 weekly averages.

Why is my HRV week "below my band" when it is only 1 ms lower?#

The band's edges are fixed by your own spread, so a week very close to an edge can fall just outside. The card then says "Slightly below your band", and when the printed numbers are level it says "At the lower edge of your band". A week just outside is not an alarm: what counts is whether it holds.

My Nervous System is low but my HRV looks fine. Why?#

HRV carries up to 45 % of it (25 %, 12 % and 8 %). The rest is stress, resting heart rate, the lowest night rate and morning Body Battery. Open the sheet and read "Built from": the part with the lowest value, or a trend nudge of -5 points for a steep fall, is usually the reason.

Why is my Nervous System missing?#

It needs overnight HRV or a resting rate on at least 3 days, a reading in the last 7 days, and at least one part that measures variability. A device that reports only a resting rate and a lowest night rate cannot give it. If the card says "No recent data", your readings are older than a week.

Why did my Cardiac Fitness change when my fitness did not?#

It reads the resting pulse and HRV of the last 7 days, the steadiness of the week and the lowest night pulse, and the newest morning shifts the pulse parts by up to 12 points. A hard session, a short night, alcohol or the start of an infection raise the pulse for a few days. It is a fitness indicator, not a measurement of your heart.

Does a low HRV mean I am ill?#

No. A low week can come with a hard block of training, poor sleep, alcohol, stress or an infection, and HRV cannot tell which. When it stays under your band for two weeks without more training, the coach says that training is unlikely to be the cause and points you to sleep, stress or a starting illness. If you feel unwell, rest and see a doctor if it does not ease.

Can I change the band or the weights?#

No. There is no setting for them. A change to a weight or a threshold changes every day's number, so we change them only in releases, and these entries are checked against the code each time.