Waking up does not trigger an increased release of the stress hormone cortisol. However, cortisol does rise in the hours before waking up as part of the body’s preparation for the next day, a new study led by the University of Bristol has found. The study has been published in the Proceedings of the Royal Society B.

Waking Up Does Not Lead to Increased Cortisol Release

For many years, it has been assumed that waking up stimulates the release of the hormone cortisol – a phenomenon known as the ‘cortisol awakening response’ (CAR). This response has been used to study many clinical conditions, including PTSD, depression, obesity and chronic fatigue syndrome. A major problem with studies using CAR is that the protocols typically only evaluate samples taken after waking up, and not in the period before, since the measurements are taken in saliva. Consequently, the studies cannot detect a change in the rate of cortisol secretion during waking.

To settle the crucial question of whether cortisol secretion actually increases after waking up, the research team from Bristol used an automated sampling system to measure cortisol levels in the tissue of 201 healthy male and female participants aged between 18 and 68 years, both before and after waking up. The researchers found that waking up did NOT lead to an increased cortisol release, and there was no evidence of a change in the rate of cortisol rise in the hour after waking compared to the hour before waking. This suggests that changes in cortisol levels immediately after waking are much more likely to represent the end of the daily cortisol rhythm, which rises in the early hours of the morning and peaks shortly after the usual waking time.

Importantly, the study also found substantial interindividual variability in absolute concentration and rate of change, as well as differences in dynamics, which could be due to the length and timing of sleep. Based on these results, the researchers advise caution when interpreting cortisol measurements taken only in the hour after waking. The results indicate that the primary cause of changes in cortisol levels at the time of waking is primarily related to the endogenous circadian rhythm of cortisol. Furthermore, the results suggest that if cortisol is related to waking at all, it is more likely related to factors that contribute to the initiation of waking than in response to it.

Relationship Between Nighttime Cortisol Surges and Sleep

Circadian rhythms, the natural 24-hour cycles of physiological and behavioural patterns, are extremely important adaptations to life on our planet with its daily light-dark and temperature variations. Disruption of these rhythms contributes to many mental, metabolic, cardiovascular and immune health issues. Understanding the role of cortisol rhythms in many of these issues is very important for researchers to understand these disorders and their potential treatment. Stafford Lightman, Professor of Medicine at Bristol Medical School: Translational Health Sciences (THS) and one of the study’s lead authors, said: ‘Our study opens up a whole new framework for understanding the link between nighttime cortisol surges and sleep and how it can be disrupted in sleep disorders, depression, and many other medical conditions.’

By measuring both before and after waking up, this study provides much-needed and crucial insights into the dynamics of cortisol in relation to sleep and endogenous rhythms, according to the researchers, and also shows that findings that are considered generally accepted in the research community may be wrong. The research team suggests that future studies on the mechanisms of arousal from both overnight and morning sleep should carefully consider the dynamic changes in the activity of the hypothalamic-pituitary axis – the system in the body that regulates the stress response and the release of cortisol – in addition to sleep and behaviour.