Sthenos

Do you know how much energy you actually have left right now? Sthenos turns your Apple Watch into a live energy score — a single number from 0 to 100 that reflects how recharged or depleted your body is, and whether you're charging or draining at this moment. No guesswork. No manual logging. Just honest, science-backed insight from the sensors you already wear. (Sthenos was previously called Body Battery — same app and data, a sharper identity.) HOW IT WORKS Sthenos isn't a daily snapshot or a one-off rating. It's a running energy level, updated through the day: • You recharge during genuine rest and sleep — when your heart rate settles and your HRV rises relative to your own baseline. • You drain from activity, elevated heart rate, and physiological stress. • Recent training still counts — a hard workout keeps weighing on you for days, so you can tell when you're truly recovered. Everything is judged against YOUR personal baseline, not population averages. Sthenos learns your normal (about two weeks — faster if you've already been wearing your watch) and calibrates the score to your body. WHAT YOU GET • Live Energy Score — a clean 0–100 gauge that updates as new data arrives. • Energy Flow — a live read on whether you're charging or draining this hour, and why. • Sleep, Recovery & Heart Rate — how last night's sleep, your HRV, and resting heart rate compare to your baseline. • Training Load & Fatigue — workouts add load that decays over days, so overtraining shows up early. • Trends — your energy across the day and weeks; the charts always match your live score. • On-Device AI Insights — a short, plain-language read on your day, generated on your iPhone with Apple Intelligence. • Apple Watch App & Complication — glance at your score from your wrist. • Home & Lock Screen Widgets — keep your energy in view without unlocking your phone. • The Science, In-App — an About tab explains how your score is calculated, with links to peer-reviewed research. PRIVATE BY DESIGN Your health data never leaves your device. Sthenos reads from Apple Health and computes everything locally — no account, no tracking, no ads, no servers. Even the AI insights run on-device. WHAT IT READS (read-only, from Apple Health) • Heart Rate Variability (HRV) • Resting Heart Rate • Heart Rate • Sleep • Workouts REQUIRES • iPhone with iOS 18 or later • Apple Watch (Series 4 or later, watchOS 9+) worn regularly, including overnight • On-device AI Insights require iOS 26 and an Apple-Intelligence-capable iPhone Sthenos is a personal wellness tool. It is not a medical device and does not provide medical advice. Apple Watch HRV is suitable for personal trends and is not clinical-grade. Always consult a healthcare professional for medical concerns.