For many years it has been generally accepted that waking up results in a stimulus to release hormone cortisol  - a phenomenon called the “cortisol awakening response” (CAR). This response has been used to investigate many clinical conditions including PTSD, depression, obesity, and chronic fatigue syndrome (ME/CFS).

A major limitation of studies using CAR is that protocols typically only assess samples obtained after waking up, and not in the period prior to this, since measurements are taken in saliva. Consequently, the studies are not able to prove a change in the rate of cortisol secretion over the awakening period.

To resolve the critical question of whether the rate of cortisol secretion actually increases after wakening, the Bristol research team used an automated sampling system to measure tissue cortisol levels both before and after wakening in 201 healthy male and female participants aged between 18 to 68 years old.

The researchers found awakening did NOT result in an increase in cortisol release, with no evidence for a change in the rate of cortisol increase in the hour after waking when compared with the hour prior to waking. This suggests that any change in cortisol levels immediately after waking are much more likely to be the tail end of the daily rhythm of cortisol - which starts increasing in the early hours of the morning, and reach a peak shortly after habitual wake time.

Read the full University of Bristol news item

Paper

‘Awakening not associated with an increased rate of cortisol secretion’ by Stafford L. Lightman, Samantha Klaas, Thomas J. Upton et al. in Proceedings of the Royal Society B [open access]