Fatigue: An Experimental Energy/Stress Index
Estimated from a single camera (PPG) reading — an educational estimate, not a medical measurement. State labels (Flow, Focus, Alarm…) are rough, experimental patterns, not emotion detection.
What this number is
Fatigue on this page is an experimental index. It is not a measure of sleepiness, muscle fatigue or overtraining. It is computed from two other indices in the tool, Energy and Stress, so it rises when your heart rate is above about 75 bpm and when your beat-to-beat rhythm looks irregular.
How this tool calculates it
Fatigue = 60 × (1 − Energy/100) + 0.2 × Stress, limited to 0–100%. Energy falls as heart rate climbs above 75 bpm and as CSI (the irregularity of your beat intervals) rises; Stress is CSI × 130. The tool works on a rolling 15-second window of fingertip video (about 30 frames per second), finds each pulse peak, keeps only beat-to-beat (RR) intervals between 400 and 1,500 ms that are within 22% of the median, and smooths the displayed numbers so they change gradually.
How to read it
Higher
Your pulse was elevated and/or your beat intervals varied a lot relative to their average. Being tired can do this, but so can coffee, having just climbed stairs, talking, or a noisy camera signal.
Lower
Heart rate was moderate and the rhythm looked steady during the reading.
Important limits
This is an experimental heuristic built from a few camera-derived inputs. It is not a validated physiological measure, it is not emotion detection, and it is not a medical test. Treat it as a rough hint about the direction your signal is moving, not as a verdict about how you feel. Real fatigue assessment usually looks at trends over days (resting heart rate, HRV measured the same way each morning, sleep), not a single short reading.
What actually helps
Consistent sleep
Regular bed and wake times are one of the most dependable ways to feel less worn out.
Track trends, not moments
If you want to monitor recovery, measure at the same time each day, sitting still, and look at the week rather than one number.
Easy movement and breaks
A short walk or a few minutes of slow breathing can help you reset during a long day.
The ONDA app is a separate product: it focuses on pulse, breathing and, with an Apple Watch, coherence. It does not use the formulas on this page.
The ONDA Principle
A high Fatigue value is a prompt to check the basics (sleep, caffeine, recent activity), not a diagnosis.
Ready to measure?
The ONDA app works with your iPhone camera, Apple Watch and Apple Health.