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.

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
• The heart gauge — your score fills a heart that empties as you deplete and refills as you recover. The number sits inside it and inverts where the waterline crosses, and it beats at your own resting heart rate.
• Energy flow — a live read on whether you're charging or draining this hour, and why.
• Sleep, Recovery, Heart and Freshness — the four things moving your score, each one tappable for a plain-English explanation of what it means today.
• Trends — your history drawn as a tide, at a day, a month, three months or six, from the same model as the live score.
• Training load — Fitness, Fatigue and Form from the training impulse of every workout, with the raw daily load drawn underneath, plus the acute:chronic ratio and its sweet spot.
• Vitals — every reading your watch takes, each on a rail showing your own normal range and where today sits: heart rate, resting HR, HRV, blood oxygen, respiratory rate, VO₂ max, steps, active energy, sleep, wrist temperature, weight.
• On-device AI insights — a short, plain-language read on your day, generated on your iPhone with Apple Intelligence.
• Apple Watch app and complications — your score, factors, vitals and training load on the wrist, plus the score as a filling heart, a training-load complication, and a configurable single metric.
• Home-screen widgets — a score widget, plus two configurable Vitals widgets where you choose every metric shown.
• The science, in-app — an About tab explaining how the score is calculated, with links to the research, and saying plainly where the evidence is contested.

PRIVATE BY DESIGN
Your health data never leaves your device. Sthenos reads from Apple Health and computes everything locally — no account, no sign-up, no tracking, no ads, no servers. Even the AI insights run on-device.

WHAT IT READS (read-only, from Apple Health)
• For the score: HRV, resting heart rate, heart rate, sleep, workouts
• For Vitals: blood oxygen, respiratory rate, VO₂ max, steps, active energy, wrist temperature, body mass
• For the optional fitness-age card: date of birth, biological sex, height, body composition, exercise and stand time, walking heart rate, mobility measures, daylight, blood glucose and pressure

Nothing is written back to Apple Health.

REQUIRES
• iPhone with iOS 18 or later
• Apple Watch (Series 4 or later) worn regularly, including overnight, for HRV and sleep. The watch app needs watchOS 11.
• 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.