Accuracy Assessment of Oura Ring Nocturnal Heart Rate and Heart Rate Variability in Comparison With Electrocardiography in Time and Frequency Domains: Comprehensive Analysis. - Cao, Rui; Azimi, Iman; Sarhaddi, Fatemeh; Niela-Vilen, Hannakaisa; Axelin, Anna; Liljeberg, Pasi; Rahmani, Amir M (Journal of Medical Internet Research, 2022)