Chapter 6 172 current study, questions were added to the CSD which inquired about experiences using the sleep tracker. A comment field was added in which participants could note comments about the measurement night, including remarks on the sleep tracker. To assess sleep quality and symptoms of insomnia, the Pittsburgh Sleep Quality Index (PSQI) and Insomnia Severity Index (ISI) were used at every measurement. The PSQI is a 19-item questionnaire that broadly assesses components of sleep quality, including sleep duration, sleep quality, sleep disturbances and daily burden of sleep problems (Buysse et al., 1989). It is scored into seven subscores, which are then added up to form a sum score (range 0 to 21). The PSQI has an established cutoff score of 5, with a score of 5 or lower indicating “good” sleep and a score higher than 5 indicating “poor” sleep. The ISI is a seven-item questionnaire that inquires about symptoms of insomnia in the previous two weeks (Bastien et al., 2001). The seven items are all scored from 0 to 4, resulting in a sum score from 0 to 28. There are clinical cutoffs for the ISI scores, with a score below 7 indicating no presence of insomnia, 8 to 14 indicating subclinical insomnia, 15 to 21 indicating clinical insomnia and a score over 21 indicating severe insomnia. The items in the ISI correspond to the DSM-5 defined criteria for clinical insomnia, and is a widely used tool to assess the burden of insomnia symptoms. To assess perceived stress and depressive symptoms, the Perceived Stress Scale (PSS; Cohen et al., 1983) and Inventory of Depressive Symptomatology-Self Report (IDS-SR; Rush et al., 1996) were used. Questionnaire results for both were used to assess the presence of selection bias in the sleep EEG measurements as displayed in the supplementary materials in Table S6.2. Sleep architecture Sleep architecture, consisting of Sleep Onset Latency (SOL), Total Sleep Time (TST), Wake After Sleep Onset (WASO), Sleep Efficiency (SE), Slow Wave Sleep duration (SWS), Number of Arousals (NRA), Number of Interruptions (NRI; > 5 minutes awake), REM sleep duration and REM sleep latency was measured using the EEG sleep measurement device with an embedded automated sleep stager. This sleep measurement device uses dry single-lead EEG for measuring sleep, with electrodes at Fpz and M1. Based on this single channel EEG signal, the embedded stager can differentiate wakefulness, light sleep (i.e. phase 1 and 2), deep sleep (i.e. phase 3 sleep or SWS) and REM sleep, with excellent performance in validation studies compared to
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