Measurement versus Inference in Sleep Assessment: Polysomnography and the Oura Ring Compared

Sleep stages are defined by cortical electrophysiology. Under the rules of the American Academy of Sleep Medicine (AASM), wake, N1, N2, N3 and REM are scored from electroencephalography (EEG), electrooculography (EOG) and chin electromyography (EMG). The Oura Ring records none of these signals. It records a peripheral pulse waveform, finger motion, finger skin temperature and reflectance oximetry, and it maps these signals onto sleep stages with a proprietary machine-learning classifier. Its sleep stages are therefore inferred, not measured.

This paper compares the two systems signal by signal and output by output, examines what the widely publicised "95% accuracy" figure actually represents, and assesses the independence of the evidence base.