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What Is the Fatigue Index? How to Objectively Measure Physical Exhaustion

Q: Three core metrics measuring recovery level

HRV (Heart Rate Variability): The variation between heartbeats. High HRV = a flexible, well-recovering nervous system. Consistently low HRV = body under high stress or not recovered. Resting heart rate: Morning resting heart rate 5–10 bpm above your baseline signals the body needs more recovery time. Deep sleep: The deep sleep stage (slow-wave sleep) is when muscles recover and growth hormone is secreted most.

Q: Why subjective feelings are often unreliable

Research consistently shows people misjudge their own fatigue levels and performance impairment — especially with chronic sleep deprivation. After several days of sleep deprivation, the brain adapts to the tired state and "normalizes" it — you no longer feel as tired as initially, but performance continues to decline. This is why objective device metrics matter more than feelings.

Q: Improve recovery metrics in 7 days

Consistent sleep timing (same bedtime and wake time) has the fastest impact — HRV often improves and resting heart rate drops within 5–7 days. Reducing alcohol and caffeine after 2 PM noticeably improves deep sleep. Morning moderate exercise is better than evening exercise for HRV. Tracking these metrics daily through H-Core shows real progress — not just relying on how you feel.

Fatigue is subjective — "tired" for one person may be normal for another. The fatigue index is a combination of objective physiological markers: HRV (heart rate variability), resting heart rate, deep sleep quality, and recovery trends over time. These metrics reflect the true state of your autonomic nervous system and body's recovery capacity.

Reference ranges

Good recoveryHigh HRV, resting HR < 60, deep sleep > 20%

Average recoveryAverage HRV, resting HR 60–75

Poor recovery — Rest neededLow HRV, resting HR > 75, deep sleep < 10%

* Reference values only. Consult a doctor for personalized advice.

Three core metrics measuring recovery level

HRV (Heart Rate Variability): The variation between heartbeats. High HRV = a flexible, well-recovering nervous system. Consistently low HRV = body under high stress or not recovered. Resting heart rate: Morning resting heart rate 5–10 bpm above your baseline signals the body needs more recovery time. Deep sleep: The deep sleep stage (slow-wave sleep) is when muscles recover and growth hormone is secreted most.

Why subjective feelings are often unreliable

Research consistently shows people misjudge their own fatigue levels and performance impairment — especially with chronic sleep deprivation. After several days of sleep deprivation, the brain adapts to the tired state and "normalizes" it — you no longer feel as tired as initially, but performance continues to decline. This is why objective device metrics matter more than feelings.

Improve recovery metrics in 7 days

Consistent sleep timing (same bedtime and wake time) has the fastest impact — HRV often improves and resting heart rate drops within 5–7 days. Reducing alcohol and caffeine after 2 PM noticeably improves deep sleep. Morning moderate exercise is better than evening exercise for HRV. Tracking these metrics daily through H-Core shows real progress — not just relying on how you feel.

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What Is the Fatigue Index? How to Objectively Measure Physical Exhaustion

Reference ranges

Three core metrics measuring recovery level

Why subjective feelings are often unreliable

Improve recovery metrics in 7 days

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