Circadian Cortisol Rhythm Measurement

Over the course of 2 days, participants provided saliva samples using Salivette kits (Sarstedt, Germany) upon waking, 30 min after waking, midafternoon (~1500 hours) and 30 min before bedtime. Samples were stored frozen at −20 °C until assayed by luminescence immunoassay reagents according to commercial instructions (Immuno-Biological Laboratories, Hamburg, Germany). Total daily cortisol production was calculated as the area under the curve with respect to ground (AUCg)^{20 (link)} using exact time of sample collection. Outlying values (+2 s.d.) were corrected using winsorizing in the larger sample from the parent study (N=135).^{19 (link)}