Behind the Numbers: How Accurate Are Smartwatch Health Sensors?

Smartwatches are now used for more than notifications; they're tools people rely on for health insights. Understanding how sensors work and what affects their accuracy helps you interpret data and make smarter health decisions.

Common sensors explained

Most modern watches include a combination of:

• Optical heart rate (PPG): Uses LEDs and photodiodes to estimate blood volume changes at the skin surface.
• ECG: Measures electrical activity and can detect specific arrhythmias when paired with proper software.
• SpO2: Estimates blood oxygen saturation through multispectral light absorption.
• Accelerometer/gyroscope: Tracks movement for step counting and activity recognition.

Factors that affect accuracy

Accuracy depends on sensor quality, firmware algorithms, fit, and user characteristics:

• Fit and placement: A loose watch yields poor PPG readings. For consistent results, wear the device snugly above the wrist bone.
• Skin tone and tattoos: Darker skin tones and tattoos can affect light absorption and scattering, impacting PPG accuracy.
• Motion artifacts: During high‑intensity interval training, motion can corrupt optical signals. Chest straps remain more reliable for intense activity.
• Ambient temperature: Cold skin can reduce perfusion, making measurements less reliable.

Interpreting heart rate data

For resting and steady‑state workouts, modern optical sensors are typically within an acceptable range of chest straps. During rapid intervals, optical readings can lag or under‑report peaks. Use optical HR for trends and daily monitoring, but for precise interval training rely on validated external sensors when possible.

ECG and SpO2 — clinical vs consumer use

ECG features detect certain patterns (like AFib) but are not a replacement for clinical evaluation. SpO2 is most useful for trend monitoring (e.g., detecting sleep‑related drops) rather than diagnosing conditions. Any concerning readings should trigger a conversation with a healthcare professional.

Algorithm improvements matter

Firmware updates often improve accuracy substantially. Manufacturers use large labeled datasets to refine algorithms; therefore, older watches receiving algorithm updates can improve over time. That makes long‑term software support a key consideration when choosing a device.

Practical tips for users

• Ensure a proper fit: tight enough to eliminate light leakage but comfortable.
• Calibrate with known references: compare occasional workouts with a chest strap or clinical device.
• Update firmware regularly for improved algorithms.
• Use multiple data points: combine sleep, HRV, and resting HR trends to make decisions.

Final note

Smartwatch sensors are powerful tools for awareness and trend monitoring. They’re best used as early warning systems and lifestyle guides — not standalone medical diagnostics. When used with proper context and occasional verification, they provide meaningful insights that can improve health outcomes.

By Dr. Samuel Lee • 2024-06-15