Saliva samples were collected in hourly intervals during the course of the day under all three light conditions. The first saliva sample was taken in the morning at 9:00 a.m., immediately after the start of the respective light exposure. The last saliva sample was collected towards the end of the test day at 5:00 p.m. The preparation of the cortisol saliva samples was carried out in the in-house laboratory of the University Psychiatric Clinics in Basel. Prior to the measurement of cortisol, the saliva sample was vortexed and centrifuged at 1500 x g for 15 minutes to remove the precipitate. According to the manufacturer’s protocol, the cortisol concentration was determined using the high-sensitivity salivary cortisol enzyme immunoassay kit (Salimetrics Europe, UK) [51 (link), 52 (link)]. The optical density was detected at 450 nm using the Cytation 3 Cell Imaging Multimode Reader (BioTek). The assay is a competitive immunoassay where the detected signal is inversely proportional to the amount of cortisol present in the sample. The cortisol ELISA had a sensitivity of <0.007 ug/dL, an intra-assay CV of 4.6% and an inter-assay CV of 6%.
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