Biometrics
Biometric Results
Understanding PNS, SNS, and Stress Index readings in your &you™ App.
PNS/SNS requires sufficient session length and strong signal quality (SNR). If either condition isn’t met, the algorithm can’t compute a reliable ANS estimate.
To improve reliability
- Wear the band snugly (poor skin contact is the most common cause)
- Minimize motion, especially at session start/end
- Use longer sessions
- Confirm SNR is above ~8–10 dB on the biometrics screen
How to read PNS and SNS
The ANS (autonomic nervous system) has two branches that work in balance — not in direct competition, but reflecting different physiological states.
Parasympathetic nervous system (PNS)
Higher is better (when at rest)
Controls rest, digestion, and cellular repair. Higher PNS at rest means the body is efficiently recovering — better sleep quality, lower inflammation, faster adaptation to training.
Sympathetic nervous system (SNS)
Lower is better (when at rest)
Drives alertness and exertion readiness. Chronically elevated SNS at rest signals ongoing stress from poor sleep, overtraining, illness, or emotional load — and impairs recovery.
To improve your numbers
- Prioritize consistent sleep (biggest lever)
- Zone 2 aerobic training (builds long-term PNS tone)
- Slow nasal breathing/breathwork (activates vagus nerve)
- Reduce evening stimulants (caffeine, alcohol)
- Allow adequate recovery between hard efforts
What Stress Index measures
Stress Index quantifies autonomic regulatory load from the shape and distribution of your BBI (beat-to-beat interval) data — specifically variance and skewness. A wide, flexible distribution means a low Stress Index; a narrow, rigid one means high. It correlates inversely with HRV: high HRV leads to a low Stress Index.
Like PNS/SNS, it requires a minimum window of clean, contiguous BBI data — short sessions, motion artifacts, or poor skin contact can all prevent it from appearing. The same reliability tips apply.
Stress Index ranges
Nominal resting range 5 – 15
- < 5 Very low load
- Rare. Deep relaxation or very high parasympathetic tone. Typically only elite endurance athletes at full rest.
- 5 – 15 Nominal
- Healthy resting baseline. Autonomic system is well-regulated with no meaningful stress load.
- 15 +* Mildly elevated
- Light stress or suboptimal recovery. Normal after hard training, poor sleep, or a late meal — not concerning if occasional.
Key principle: trends across multiple sessions matter more than any single reading. Interpret each result in the context of recent training load, sleep, and lifestyle stressors.
This guide is general information, not medical advice. Talk to your clinician about your own results.