Research / Meta-Analysis
Wearable
Accuracy vs PSG.
The Illusion of Sleep Staging
Consumer wearables rely almost entirely on Photoplethysmography (PPG) to measure heart rate variability (HRV) and 3-axis accelerometers for actigraphy. While useful for estimating Total Sleep Time (TST), they are fundamentally incapable of measuring cortical activity.
The REM Sleep Problem
Because wearables cannot measure muscle atonia via EMG or rapid eye movements via EOG, they infer REM sleep from autonomic nervous system changes (e.g., erratic heart rate). This leads to massive misclassification rates, often confusing light N1 sleep with REM.