Why is the LF/HF ratio in wearables not a reliable measure of stress?

Many wearables estimate stress by dividing low-frequency HRV power by high-frequency HRV power, but the article explains why this LF/HF ratio does not reliably represent stress or autonomic balance. LF power is influenced by more than sympathetic activity, while HF power can change substantially with breathing, and both values may also shift with posture, temperature, recording length, filtering, and other technical factors. Because the ratio can rise or fall even when a person’s emotional stress has not changed, using it as a standalone stress score may produce misleading results. A more meaningful assessment requires context and multiple physiological signals, such as time-domain HRV measures, respiration, skin conductance, temperature, recovery patterns, and changes observed over time.