HRV Science

Methods

108 healthy adults, randomized controlled trial, 4 intervention arms, 28 days

Key Findings

• Cyclic sighing increased RMSSD by approximately 12%
• Positive mood improvements persisted even on non-practice days
• Box breathing showed smaller but still significant improvements
• Mindfulness meditation was least effective for HRV specifically

Limitations

Self-reported compliance, predominantly young healthy adults, no long-term follow-up

Methods

Review of biofeedback literature and clinical protocols

Key Findings

• Resonance frequency is typically 4.5-7 breaths per minute for most adults
• Breathing at resonance frequency maximizes respiratory sinus arrhythmia
• Effects are immediate and measurable within a single session
• Regular practice leads to sustained improvements in baseline HRV

Limitations

Individual resonance frequency requires testing to determine precisely

Methods

Systematic review of 45 consumer wearable validation studies

Key Findings

• PPG-derived RMSSD averages 6-11% lower than ECG measurements
• Correlation with ECG remains strong (r > 0.9) for most devices
• Only 13 of 45 devices reviewed had FDA approval
• Motion artifacts remain the biggest accuracy challenge

Limitations

Studies used different protocols, making direct comparison difficult

Methods

Analysis of continuous HRV data from Oura Ring users over multiple weeks

Key Findings

• 5+ nights of data needed for reliable HRV-CV estimation
• Higher HRV-CV associated with alcohol consumption
• Higher HRV-CV linked to lower physical activity levels
• Higher HRV-CV correlated with inconsistent sleep schedules

Limitations

Observational data, correlation not causation

Methods

Analysis of wearable HRV data correlated with self-reported alcohol intake

Key Findings

• 1-2 drinks suppress HRV for approximately 2 days
• 3+ drinks can affect HRV for 4-5 days
• Effect is dose-dependent and cumulative
• Sleep quality also impaired during recovery period

Limitations

Self-reported alcohol intake may be underestimated

Methods

Meta-analysis of 15 RCTs with HRV outcomes

Key Findings

• 1-2g daily EPA/DHA showed most consistent benefits
• RMSSD improved by 5-15% on average
• Benefits more pronounced in those with lower baseline omega-3 status
• Effects take 8-12 weeks to fully manifest

Limitations

Heterogeneous study designs, varying doses and formulations