Waking up does not make you stressed, study finds

There is no evidence to suggest a rise in the rate of the stress hormone cortisol in the hour after waking

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Waking up does not increase the release of the stress hormone cortisol into the bloodstream, but it does rise in the hours before waking to prepare the body for the day ahead, new research led by the University of Bristol has found.

In the past, scientists have believed that waking up stimulates the release of cortisol in a phenomenon called the “cortisol awakening response”, or CAR.

Studies into CAR have been limited as they measured saliva in samples taken after waking up, and not in the period before. The studies were not able to prove a change in the rate of cortisol secretion over the awakening period.

To resolve this, the research team used an automated sampling system to measure tissue cortisol levels before and after waking in 201 healthy male and female participants aged between 18 and 68 years old.

“By measuring both before and after waking, this study provides much needed and crucial insight into the dynamics of cortisol with respect to sleep and endogenous rhythms," said Dr Thomas Upton, clinical research fellow and co-lead author in the Bristol Medical School Translational Health Sciences.

"For me, a key message is that much caution should be exercised if attempting to interpret post-wake cortisol values where information about the pre-waking state is not known.”

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In the study published today in the Proceedings of the Royal Society B, the researchers found waking did not result in an increase in cortisol release and there was also no evidence to suggest a rise in the rate the hour after waking, compared with the hour before waking.

“Our study opens up a whole new framework for understanding the relationship of overnight increases in cortisol with sleep, and how this may be disrupted in sleep disorders, depression and many other conditions," Stafford Lightman, another professor of medicine at the same school, and a lead author of the study.

The team 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 reaches a peak shortly after normal waking time.

The findings show that the major cause of any changes in cortisol about the time of awakening are mainly related to the endogenous circadian rhythm of the hormone. Results also suggest that if cortisol has any relationship to waking, it is with factors that contribute to waking rather than a response to it.

Disturbances of circadian rhythms, the natural 24-hour cycles of physiological and behavioural patterns, contribute to many psychological, metabolic, cardiovascular and immunological health conditions. Understanding the role of cortisol rhythms in many of these conditions will be very important for researchers looking to treat these disorders in the future.

“As well as providing important insights into the biology of our sleep-wake cycles, this work illustrates how findings that have become received wisdom within the research community may be wrong," said Marcus Munafo, professor of biological psychology and associate pro vice chancellor for research at the University of Bristol.

The research team suggest that future studies should carefully consider 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.