Free cortisol awakening responses are influenced by awakening time

Psychobiological investigations on the hypothalamus–pituitary–adrenal (HPA) axis depend on markers that adequately describe the activity of this system. There is evidence that the free cortisol response to awakening, proposed as a marker for the HPA axis, can be influenced by time of awakening. To further investigate this possible confounder, 24 shift working nurses and 31 female students on a regular sleep–wake cycle collected saliva samples 0, 30, 45 and 60 minutes after awakening. Nurses were investigated on the first and second day of their early (awakening: 0400–0530 h), late (awakening: 0600–0900 h), and night shift (awakening: 1100–1400 h), respectively. Students were studied after taking a short nap on two consecutive weekdays (awakening: 1845–2030 h). Mean cortisol levels after awakening increased significantly under all three shift conditions (p<0.01), but decreased in the student sample (p<.05). Within the three shift conditions, cortisol responses following waking in the early shift were more pronounced than in late (p<.01) and night shift (p<.05). The present study shows that in a sample with a large range of awakening times, an impact of this variable on the cortisol awakening response can be observed. The data furthermore strongly suggest that waking up per se is insufficient for adrenocortical stimulation