How EveryBand measures HRV

Heart-rate variability describes variation in the time between heartbeats. EveryBand distinguishes the method behind a value: Health HRV (SDNN) and a local nightly RMSSD estimate are different measurements. They should not be compared as though they were the same number.

The current documented implementation can display available Apple Health HRV while a separate nightly RMSSD estimate remains unavailable. Enabling Health access does not convert SDNN into RMSSD or guarantee a Recovery score.

These methods were reviewed on 15 October 2026. The installed beta may differ, and measurement availability depends on the source, firmware, permissions and retained data. EveryBand is experimental wellness software and is not a medical device.

SDNN summarizes the spread of accepted intervals around their average. RMSSD emphasizes differences between successive accepted intervals. Both are time-domain HRV methods, but their values depend on recording length, filtering and measurement context. The ESC/NASPE standards provide the terminology; they do not validate EveryBand's acquisition or scores.

Apple exposes the SDNN HRV quantity through HealthKit. A value displayed locally as RMSSD is not renamed SDNN to fit that category.

The native history path first checks supported record formats, source identity, interval units and plausible interval values. Conflicting records, explicitly unworn observations and unsupported interval positions are rejected. Accepted intervals must fall within the selected sleep context with the required heart-rate and motion evidence.

The calculation compares accepted adjacent recorded intervals within five-minute windows. It rejects an adjacent jump greater than 20% of the pair's average. Each usable window needs at least 100 accepted adjacent differences. The night needs at least three usable windows, with the earliest and latest at least six five-minute positions apart.

For each usable window, RMSSD is the square root of the average squared difference between accepted adjacent intervals. The nightly result is the median of those window estimates. Using short windows avoids treating a whole night, including changing sleep context, as one uninterrupted sequence.

Too little retained interval data, an incomplete sleep window, rejected records or insufficient coverage can leave local RMSSD unavailable. A heart-rate trace alone is not enough to create beat-to-beat variability. A Full purchase supplies access to features; it does not supply missing sensor evidence.

For Health HRV, first check that the originating device or app actually records the category and that EveryBand has permission to read it. Source timing also matters: a record that has not reached Health cannot appear in EveryBand. See the Apple Watch recovery guide for the difference between having Health measurements and meeting a score's inputs.

The native ordinal-based estimate does not establish exact beat timestamps or verified skin contact. It is not an ECG, a qualified Health heartbeat series or, by itself, a qualified SDNN export. Experimental interval variation also does not automatically qualify as either HRV method.

A higher number is not a diagnosis, a guarantee of recovery or a reason to disregard symptoms. Fit, motion, missed observations and algorithm revisions affect the result. Scores from other apps can differ because their sampling, filtering and formulas differ; EveryBand makes no accuracy-superiority claim.

EveryBand is independent and is not affiliated with, endorsed by or sponsored by WHOOP or Apple. Their names identify compatible hardware and data services.

Written by EveryBand. Published October 16, 2026; reviewed October 16, 2026. Product descriptions are subject to the installed build; third-party features and terms may change.

• ESC/NASPE Task Force: Heart rate variability standards
• Apple HealthKit: heartRateVariabilitySDNN

EveryBand is an independent app and is not affiliated with, endorsed by, or sponsored by WHOOP, Apple, or Bevel. Third-party product names and trademarks belong to their respective owners and are used only for factual compatibility and interoperability descriptions.

Other app and device names belong to their owners. EveryBand is also independent of Sixteen and its publisher. For corrections, contact EveryBand.

• How the estimates workRead the full explanation.
• How EveryBand calculates RecoveryEveryBand Recovery combines nightly RMSSD, resting heart rate and Sleep with personal history. It needs the current night and two prior complete baseline nights.
• How EveryBand calculates SleepEveryBand Sleep combines duration against estimated need, regularity, efficiency and sleep stress. Stages and missing data have separate limits.
• Apple Watch recovery: what your Health data can showUnderstand Apple Watch data in EveryBand, why Health HRV differs from local nightly HRV, and when Recovery can remain unavailable.
• Data sources and compatibilityRead the full explanation.