The physiological stress response is larger in the morning than evening

The real-world implications are far from clear and the result needs to be replicated with larger samples

By Emma Young

When’s the best time of day to give someone bad news? First thing in the morning or early evening? Yes, if it’s in the morning, they have longer to work out what to do about it, but you might be better off plumping for the evening because according to a new study, published open-access in Neuropsychopharmacology, they’re likely to suffer less of a physiological stress response at this time. 

If a threat – whether physical or psychological – doesn’t quickly vanish, communication between a trio of brain regions – the hypothalamus, the pituitary glands and the adrenal cortex (known collectively as the HPA axis) – causes, among other things, an increase in levels of the hormone cortisol, which triggers the release of glucose, for energy, into the bloodstream. This stress response effectively provides our muscles with extra fuel to fight or flee. 

However, there’s also a daily pattern to our baseline levels of cortisol production – healthy people typically experience a spike around waking, and a steady decrease in levels throughout the day. 

All kinds of other factors also influence an individual’s cortisol levels at any given time, including age, sex, sleep/wake time preferences, exercise patterns and general stress levels. This raises the possibility that how we respond physically to stress might vary depending on the time of day. 

To find out, Yujiro Yamanaka at Hokkaido University and colleagues recruited 20 male and 7 female healthy young subjects with similar typical sleep durations and average wake/sleep times (in other words, they were not “larks” or “owls”); also none of them had a history of hormonal, psychiatric or sleep problems. The participants first gave saliva samples every two hours for one day, so their typical daily cortisol levels could be assessed. 

They were then randomised to undergo a stressful experience either two hours after they typically woke up, or ten hours afterwards. For the stress experience, the researchers used the widely-used Trier Social Stress test, which requires people to give a presentation and perform mental arithmetic in front of interviewers, while being videoed. Saliva samples were collected from all the participants before their test, immediately afterwards and then every ten minutes for half an hour. 

The researchers found that the group that were stressed in the morning showed a statistically significant increase in their cortisol levels after the test, compared with before (a sign of a large stress response). In contrast, the cortisol increase in the evening group was smaller and did not reach statistical significance. 

The researchers said it seems that the HPA axis “has a powerful response to acute psychological stress in the morning rather than in the evening”. Animal research suggests that this might be because the adrenal cortex typically becomes less sensitive to ACTH, the hormone produced by the pituitary gland that triggers the release of cortisol, as the day wears on. 

What are the implications of all this? 

Given all the factors that affect an individual’s cortisol levels and their response to stress, the real-world impact of experiencing morning vs. evening stress is far from clear. There’s another complication – the researchers explained that if stress in the evening doesn’t significantly hike cortisol levels, and so does not increase glucose availability, we might in fact be more vulnerable to any threat, because we might find it harder to mount a useful response. On the other hand, if that stressor is something we can’t immediately combat, and we’re frequently stressed, arguably it might be better to have a lower cortisol response, because chronically elevated levels of cortisol are associated with ill health. In other words, the short answer to the question of whether to share bad news in the morning or evening is: “it depends!”.

Meanwhile, for other researchers interested in stress, these new results (while they need replicating with larger samples) also have implications: the time of day of experiments investigating stress could, it seems, affect the results in important ways. 

Hypothalamic‐pituitary‐adrenal axis differentially responses to morning and evening psychological stress in healthy subjects

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest

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