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⚡ How Sthenos Works
Sthenos is a running 0–100 energy level — not a daily snapshot. It's integrated over time from the data your Apple Watch already collects: heart-rate variability (HRV), resting and ambient heart rate, sleep, and workouts. Each moment, your energy recharges during genuine rest and sleep and drains from activity, heart-rate elevation, and physiological stress, with a lingering penalty from recent training:
energy = previous + recharge − drain − training load
Sleep recharges
Sleep is the strongest recharger — a good night can add 40–70 points. Quiet daytime rest helps less.
Activity drains
Workouts, elevated heart rate, and stress pull from your reserves in real time.
Your personal baseline
HRV and resting heart rate are judged against your own rolling 7- and 28-day baselines — not population averages.
Residual fatigue
Each workout adds training load (Banister TRIMP) that decays over the following days, so a hard session still weighs on you tomorrow.
To show your level right now, the app replays roughly the last 48 hours of your Apple Watch data through this model — so it's consistent whether or not the app has been open, and the Trends charts always match the number on the Today tab. The full scoring method and scientific references are inside the app, on the About tab.
📱 Requirements
- iPhoneiOS 18 or later.
- Apple WatchSeries 4 or later (watchOS 9+), worn regularly — including overnight — for HRV, sleep, and heart-rate data. The score is only as good as the data it can read.
- Apple HealthPermission to read Heart Rate, HRV, Resting Heart Rate, Sleep, and Workouts.
- AI Insights (optional)Runs on-device via Apple Intelligence — requires iOS 26 on a compatible iPhone (15 Pro or later) with Apple Intelligence enabled.