Root tracks eight metrics pulled directly from Apple HealthKit (Apple Watch required for most biometrics). Here is each one, what it measures, its unit, and what direction signals better recovery.
Recovery Signals
HRV (Heart Rate Variability)
Unit: milliseconds (ms) · Direction: Higher is better
The variation between consecutive heartbeats during sleep. Root captures this within your actual sleep window — not a whole-day average — so it reflects the recovery that happened while you were asleep. Your personal mean typically ranges anywhere from 20 ms to 100 ms+ depending on age, fitness, and genetics. Track the trend, not the absolute number.
See Understanding HRV for a full explanation of what HRV measures and how Root’s three tiers work.
Sleeping Heart Rate
Unit: bpm · Direction: Lower is better (within healthy limits)
This is Root’s own calculation, not Apple’s standard resting heart rate. Root uses the bottom 20th percentile of heart rate samples recorded during confirmed asleep segments. Using the bottom 20% filters out brief movement spikes and arousal periods that would inflate the average, giving a truer picture of how hard your heart worked (or didn’t) during real sleep. Lower values indicate deeper parasympathetic recovery.
Vitals
Respiratory Rate
Unit: breaths per minute (br/min) · Direction: Context-dependent baseline
How many times you breathe per minute during sleep, measured by Apple Watch’s accelerometer detecting chest movement. The healthy adult range at rest is roughly 12–20 br/min, but your personal baseline matters more. An elevated respiratory rate can indicate illness, sleep-disordered breathing, or high physiological stress. A sudden increase of 2+ breaths from your norm is worth noting.
Blood Oxygen (SpO2)
Unit: percentage (%) · Direction: Higher is better
The saturation of oxygen in your blood, measured by the Watch’s optical sensor during sleep. Healthy values are typically 95–100%. Consistently low readings or large dips can be associated with sleep apnoea or altitude effects. Root tracks this overnight and flags anomalies when your value falls more than 2 standard deviations below your personal baseline.
Night’s Flow
Total Sleep
Unit: hours · Direction: More is better (up to a healthy ceiling)
The sum of all sleep stages (Deep + Core + REM) excluding awake time. Root attributes this to your wake-up date, not your fall-asleep date, using an 18:00 cutoff — sleep starting after 6 PM is counted toward the next calendar day. Most adults need 7–9 hours; consistent nights below your personal baseline flag as Recharging.
Restorative Sleep
Unit: hours · Direction: Higher is better
The combined duration of Deep sleep (slow-wave, physical repair) and REM sleep (emotional processing, memory consolidation). Core sleep bridges these stages but is considered less restorative. Root visualises all four stages in the intra-night sleep ribbon on the Night tab:
| Stage | Colour | Role |
|---|---|---|
| Deep | Dark blue | Physical repair, immune support, waste clearance |
| Core | Medium blue | Light NREM sleep between deeper stages |
| REM | Light blue | Memory consolidation, emotional processing |
| Awake | Warm yellow | Brief arousals between cycles |
Fall Asleep Time
Unit: HH:mm · Direction: Consistent is better
The clock time when you first entered sleep. Root tracks this against your baseline to detect drift — a progressively later bedtime is one of the earliest signs of a disrupted sleep schedule. Neither earlier nor later is inherently better; what matters is consistency relative to your own rhythm.
Wake Up Time
Unit: HH:mm · Direction: Consistent is better
The clock time of your final awakening. Combined with fall asleep time, it determines your sleep window — and whether it’s drifting earlier or later across days and weeks.
How Root Uses These Metrics
Each metric is shown with a range bar — a visual indicator of where tonight’s reading sits within your personal baseline range (your 30-day mean ± one standard deviation). A dot to the left of the bar means you’re below your normal range; to the right means above.
Tap the ⓘ icon on any metric card in the app to see a brief explanation. See How Your Baseline Works for a full explanation of how the range is calculated.