Your smartwatch constantly tracks your heart rate, generating mountains of data throughout the day. But what does that data actually mean? How accurate is it? And more importantly, how can you use this information to improve your health and fitness?

This guide explains the technology behind optical heart rate monitoring, helps you interpret your readings meaningfully, and clarifies when you should pay attention versus when numbers are just noise.

How Optical Heart Rate Monitoring Works

Smartwatches use a technology called photoplethysmography (PPG) to measure heart rate. While the name sounds complex, the principle is straightforward:

  • Green LED lights shine into your skin
  • When your heart beats, blood flow increases in your capillaries
  • Blood absorbs green light more than surrounding tissue
  • Photodiode sensors detect changes in reflected light
  • Software algorithms convert these light fluctuations into heart rate readings

This happens continuously throughout the day, with your watch taking measurements every few seconds (in continuous mode) or every few minutes (in battery-saving mode).

Why Green Light?

Green light is absorbed more effectively by haemoglobin in blood than other wavelengths, making changes in blood flow easier to detect. Some premium watches also use red and infrared LEDs for blood oxygen monitoring, which requires different light wavelengths to measure oxygen saturation.

Key Heart Rate Metrics Explained

Resting Heart Rate (RHR)

Your resting heart rate is measured when you're completely at rest, ideally during sleep or immediately upon waking. A typical adult RHR ranges from 60-100 beats per minute, though athletes often have RHRs in the 40-60 range due to more efficient cardiovascular systems.

Why RHR matters:

  • Fitness indicator: As cardiovascular fitness improves, RHR typically decreases
  • Recovery signal: An elevated RHR often indicates incomplete recovery, illness, stress, or poor sleep
  • Trend tracking: Consistent changes over weeks provide meaningful fitness insights

A single day's RHR reading means little—focus on trends over weeks and months. A gradual decrease suggests improving fitness; a sudden sustained increase warrants attention.

Maximum Heart Rate

Your maximum heart rate (MHR) represents the highest rate your heart can safely sustain during intense exercise. The classic formula "220 minus your age" provides a rough estimate, but individual variation is substantial. Your actual MHR might be 10-15 bpm higher or lower than this calculation suggests.

Knowing your true MHR helps you train in appropriate heart rate zones, but determining it accurately requires a maximal exercise test—not something most people need or should attempt without medical supervision.

Heart Rate Variability (HRV)

HRV measures the variation in time between consecutive heartbeats. Unlike heart rate, which counts beats, HRV examines the subtle timing differences from beat to beat.

Higher HRV generally indicates:

  • Better cardiovascular fitness
  • Good recovery status
  • Lower stress levels
  • Healthy autonomic nervous system function

Lower HRV may suggest:

  • Fatigue or overtraining
  • Illness onset
  • Elevated stress
  • Poor sleep quality

HRV Interpretation Caution

HRV is highly individual—there's no universal "good" number. What matters is your personal baseline and how your readings change over time. Many factors affect HRV, including alcohol consumption, caffeine, sleep position, and hydration. Don't panic over single low readings.

Training with Heart Rate Zones

Heart rate zones divide your exercise intensity into distinct ranges, each providing different training benefits:

  • Zone 1 (50-60% MHR): Very light activity, recovery. Walking, gentle yoga.
  • Zone 2 (60-70% MHR): Light endurance training. Easy jogging, cycling. Fat burning zone.
  • Zone 3 (70-80% MHR): Moderate aerobic training. Improved cardiovascular fitness.
  • Zone 4 (80-90% MHR): Hard effort. Threshold training, tempo runs.
  • Zone 5 (90-100% MHR): Maximum effort. Sprint intervals. Limited time sustainable.

Most recreational exercisers benefit from spending the majority of training time in Zones 2-3, with occasional Zone 4-5 efforts. This "polarised" approach builds endurance while avoiding burnout.

When to Trust (and Doubt) Your Readings

High Confidence Scenarios

  • Resting measurements when sitting or lying still
  • Steady-state cardio like jogging or cycling at constant effort
  • When the watch fits snugly and hasn't moved during measurement
  • Indoor exercise in moderate temperatures

Lower Confidence Scenarios

  • High-intensity intervals with rapid heart rate changes
  • Activities with significant wrist movement (boxing, rowing, tennis)
  • Cold weather, when blood flow to extremities is reduced
  • Immediately after starting exercise, before sensor calibration
  • When the watch band is loose or positioned incorrectly

Key Takeaway: When Readings Don't Look Right

  • If your heart rate seems impossibly high during moderate effort, the sensor may be reading your cadence (steps or pedal strokes) instead
  • If your heart rate drops suddenly mid-workout, the watch may have shifted position
  • For maximum accuracy during intense training, consider a chest strap heart rate monitor

Health Insights from Heart Rate Data

Beyond fitness tracking, consistent heart rate monitoring can provide early warning signs worth discussing with your doctor:

When to Pay Attention

  • Sustained elevated RHR: An RHR that remains significantly elevated for several days might indicate illness, stress, or another health factor
  • Irregular heart rate alerts: Some smartwatches detect irregular rhythm patterns that might warrant medical evaluation
  • Recovery patterns: If your heart rate takes much longer than usual to return to normal after exercise, you may be over-fatigued

What Smartwatches Can't Do

It's important to understand limitations. Smartwatch heart rate monitoring is a wellness feature, not a medical device. Your watch cannot:

  • Diagnose heart conditions
  • Replace medical-grade ECG monitoring
  • Detect all types of arrhythmias
  • Provide emergency health alerts with clinical reliability

If you have concerns about your heart health, consult a healthcare professional. Smartwatch data can be a useful conversation starter but should never replace proper medical evaluation.

Getting the Most from Your Data

To maximise the value of heart rate monitoring:

  • Establish your baseline: Wear your watch consistently for 2-3 weeks to understand your normal patterns
  • Track trends, not daily fluctuations: Weekly averages are more meaningful than any single reading
  • Consider context: Stress, alcohol, poor sleep, and illness all affect heart rate
  • Use the companion app: Most apps provide better data visualisation than the watch itself
  • Set meaningful alerts: High/low heart rate notifications can catch genuinely unusual events

Heart rate monitoring is one of the most valuable features of modern smartwatches—when interpreted correctly. Focus on your personal trends, maintain healthy scepticism about moment-to-moment accuracy, and let the data guide rather than dictate your wellness decisions.