Science Notes
The science behind Tuning
Plain-language explainers on HRV, SDNN and recovery — written the way we build the app: transparently.
What Is SDNN? The Heart Rate Variability Metric Your Apple Watch Already Tracks
SDNN is the most studied time-domain HRV metric. Learn what it measures, why it matters for stress and recovery, and how Apple Watch and Tuning turn it into a personal signal.
Read article →
Why Your Personal HRV Baseline Beats Population Averages
Population 'normal' HRV ranges ignore age, fitness and lifestyle. A 14-day rolling baseline built from your own Apple Watch SDNN data is a more honest signal.
Read article →
How to Improve HRV: 7 Evidence-Based Levers for Your Nervous System
HRV is trainable. Sleep, breathing, training load, alcohol, hydration and light exposure all shift your daily SDNN. Here is how to improve HRV using levers you control.
Read article →
SDNN vs RMSSD: Which HRV Metric Should You Track?
SDNN and RMSSD measure different facets of heart rate variability. Learn the difference, what each reveals about stress and recovery, and why Tuning tracks SDNN.
Read article →
HRV Normal Range by Age: Why the Charts Don't Apply to You
Searching for a normal HRV range by age? The charts vary wildly and none of them describe you. Here is what research really shows — and the only comparison that matters.
Read article →
Is Apple Watch HRV Accurate? What the Sensor Really Measures
Apple Watch estimates HRV from your pulse using light, not electrodes. Research shows good agreement with clinical references in daily conditions — and a personal baseline makes the sensor quirks irrelevant.
Read article →
How Sleep Affects HRV (and Why Your Morning Number Is Telling)
Sleep is the dominant driver of next-day HRV. Here is the mechanism, what a bad night looks like in your numbers, and how to use morning HRV as a free sleep-quality audit.
Read article →
What Is Heart Rate Variability (HRV)? The 5-Minute Guide
Heart rate variability is the variation in time between your heartbeats — and a widely studied window into stress and recovery. Here is a plain-language guide to what it is and how to use it.
Read article →
Resting Heart Rate vs HRV: Two Signals, Two Different Stories
Resting heart rate and HRV both reflect your autonomic nervous system — but they move in opposite directions and tell different stories. Here is how to read them together.
Read article →