Fitbit devices and Pixel Watch report HRV as RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD , worked out from your longest sleep in the past 24 hours, and only sleep periods longer than 3 hours count (Google Health Help). That makes it one value per night, taken from sleep, which you should compare only with your own past nights.
Documentation checked: 20 October 2026. Google updates its app often, so details can change.
What number Fitbit and Pixel Watch show
Google’s help page, which covers Pixel Watch and Fitbit devices together, says Fitbit uses the common formula called the root mean square of successive differences (RMSSD) to work out HRV from your recent heart rate data (Google Health Help). RMSSD measures how much the gap between one heartbeat and the next changes. The watch or tracker reads your pulse with an optical sensor on the wrist. The same help page covers the Fitbit Air, Google’s band with no screen; Fitbit Air HRV compares it with WHOOP.
To find it, open the Google Health app, go to the Health tab, tap Vitals under Focus areas, then Heart rate variability (HRV). The detail view shows graphs over a week, a month, a quarter or a year.
When it is measured
The latest value comes from your longest sleep period in the past 24 hours, and only sleep periods longer than 3 hours are considered (Google Health Help). Google says to wear the device for at least a full day, including to sleep at night, before you look. A nap or a short night gives no new value.
How to read it
Because the value always comes from sleep, compare it with your own past nights, and look at the weekly graph rather than one night.
HRV also feeds Daily Readiness, a score from 0 (low) to 100 (high). Google’s updated version uses heart rate variability, resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate and recent sleep, and dropped the earlier activity part. You need to wear the device for 7 nights of sleep to set a personal baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline , and Google says the baseline becomes more accurate after about a month (Google Fitbit Help). Readiness works with Pixel Watch 1 to 5 and several Fitbit models. Recovery scores explained compares it with other makers’ scores, and how long until my HRV baseline is reliable lists the waiting times.
A sleep RMSSD is not directly comparable with a 5-minute reading taken while awake. Overnight vs morning HRV weighs the two approaches.
How Fitbit and Pixel Watch compare with other devices
Neither Fitbit nor Pixel Watch was part of the 2025 Dial study, which tested Oura, WHOOP, Garmin and Polar devices against a chest-strap ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG overnight (Dial and colleagues, 2025), so that study has no Fitbit figure. One smaller study did test a Fitbit overnight.
In a 2025 pilot study, 8 people, mostly older adults (average age 64) awaiting shoulder surgery, wore a Fitbit Versa 4 and an ambulatory ECG for at least three nights. The median overnight RMSSD was 21.7 ms from the Fitbit and 20.6 ms from the ECG, not a significant difference, and the authors reported strong correlation and acceptable agreement. With so few people, they called this preliminary support (Nasr and colleagues, 2025). It says nothing about Pixel Watch or other Fitbit models.
Fitbit and Oura both report RMSSD from sleep, but each maker picks its own window and cleans the data its own way, so the numbers still differ. The HRV by device guide shows when each brand measures.
What to watch out for
- Wear it to bed every night. No sleep longer than 3 hours means no value for that day.
- Compare weeks, not nights. A single night can jump around.
- Keep the device on the same wrist, at the same tightness. That keeps your nights comparable.
- Expect a step after a new device or an update. See can a software update change my HRV. Google also says past Readiness scores Readiness score A single number, often out of 100, that a device builds from your HRV, resting heart rate, sleep and other inputs to suggest how ready you are for the day. The weighting is not published. More about Readiness score may change after an app update.
- Do not mix brands in one trend. A value from another device is a different series, even if it is also RMSSD.