Polysomnography (PSG) was performed using the SOMNOtouch device (SOMNOmedics GmbH, Randersacker, Germany). Electroencephalography was recorded from two main channels (C3 and C4 in the international 10–20 system of electrode placement) referenced to the contralateral mastoids. The common ground and reference electrode were placed at Fpz and Cz, respectively. Electrooculography (EOG; right and left outer canthi) and submental electromyography (EMG) were also recorded for sleep stage classification. EEG signals were sampled at 256 Hz and impedance was kept at less than 5KΩ for EEG and below 10KΩ for EOG and EMG channels.
Data was autoscored with the updated version of Z3Score algorithm (https://z3score.com), which has been previously validated and shown to be on par with expert scorers;25 (link) along with the FASST EEG toolbox (http://www.montefiore.ulg.ac.be/~phillips/FASST.html), and visually inspected by trained technicians who were blinded to the Oura ring and actigraphic records. Scoring of the sleep was performed based on the American Academy of Sleep Medicine Manual (AASM). WASO, REM sleep, N1, N2 and N3 were calculated based on 30-second epochs according to the AASM manual criteria.26