Research from the University of Bristol reveals that waking up does not trigger an increase in cortisol, the stress hormone. Instead, levels of cortisol actually rise in the hours leading up to waking as part of the body’s daily preparation. This study was published today [15 January] in the Proceedings of the Royal Society B.
For a long time, it was believed that waking up causes an immediate surge of cortisol, known as the “cortisol awakening response” (CAR). This concept has been utilized to explore various medical conditions, including PTSD, depression, obesity, and chronic fatigue syndrome (ME/CFS).
However, a significant drawback of CAR studies is that they usually only examine cortisol levels collected after waking, missing the critical period leading up to it, due to the reliance on saliva measurements. Therefore, these studies cannot establish whether there is an actual change in cortisol secretion rates during the awakening phase.
To address the pivotal question of cortisol secretion patterns upon waking, the Bristol research team employed an automated sampling system to gauge cortisol levels both before and after awakening in 201 healthy participants, aged between 18 and 68 years.
The findings indicated that waking did NOT lead to an increase in cortisol levels, with no significant change observed in the hour following waking compared to the hour before. This implies that variations in cortisol levels right after waking are more likely part of the established daily cortisol rhythm, which begins to rise in the early morning hours and peaks shortly after usual wake-up time.
Notably, the study revealed considerable differences in cortisol concentration and changes among individuals, which might be linked to variations in sleep duration and timing. Based on these results, the researchers caution against solely interpreting cortisol levels measured within the hour of awakening.
The study underscores that significant changes in cortisol around wake time are primarily influenced by the natural circadian rhythm rather than being a direct reaction to awakening. Furthermore, if cortisol does play a role related to waking, it is more connected with the factors that initiate waking rather than being a response to it.
Circadian rhythms, the regular 24-hour cycles of physiological and behavioral change, are crucial adaptations that help us live in sync with the daily light and temperature cycles. Disruptions to these rhythms can contribute to many health issues, including psychological, metabolic, cardiovascular, and immune conditions. Understanding cortisol rhythms is vital for researchers examining these disorders and exploring treatments.
Stafford Lightman, Professor of Medicine at Bristol Medical School: Translational Health Sciences (THS) and a lead author of the study, stated: “Our research opens up a new understanding of how overnight spikes in cortisol relate to sleep, and how this association may be affected by sleep disorders and depression.”
Dr. Thomas Upton, Clinical Research Fellow and co-lead author at Bristol Medical School (THS), added: “By evaluating cortisol levels both before and after waking, this research sheds light on the relationship between cortisol, sleep, and endogenous rhythms. A critical takeaway is that interpreting cortisol levels measured post-wake should be done with caution without prior context.”
Marcus Munafò, Professor of Biological Psychology and Associate Pro Vice-Chancellor for Research Culture at the University of Bristol, remarked: “This research not only offers significant insights into the mechanisms of our sleep-wake cycles but also emphasizes that established findings in the research community can be incorrect.”
“Ensuring the robustness and reproducibility of our research, including challenging previous conclusions, is integral to the research culture we promote at the University of Bristol.”
The research team urges that future investigations into how we awaken, both overnight and in the morning, should consider dynamic changes in the activity of the hypothalamic-pituitary axis — the body’s system that manages the stress response and cortisol release, alongside sleep and behavior.
This study received funding from the EU Horizon 2020 grant ‘Ultradian’ (grant no. 633515) and a follow-on fund from the Biotechnology and Biological Sciences Research Council (BBSRC) (BB/M019268/1).
