Lead researcher Yujiro Yamanaka, and his team studied 27 healthy young volunteers. Yamanaka first measured the participant’s diurnal rhythm of salivary cortisol levels as a baseline. The volunteers were then split into two groups. The first group was exposed to a stress test in the morning, two hours after they woke up. The second group received the same stress test in the evening, ten hours after waking.
The stress test was 15 minutes long and comprised the preparation and presentation of an interview on mental arithmetic. The interview was done via camera, in front of three trained interviewers. Yamanaka and his team took oral swabs to measure salivary cortisol levels half an hour before the test, immediately after and then at ten-minute intervals for half an hour.
Interestingly, they found that salivary cortisol levels were higher in those who took the test in the morning, while there was little to no salivary cortisol response in the group who took their stress test in the evening. The heart rates of both groups were taken, and both indicated that the sympathetic nervous system had been activated.
In the morning, the HPA axis was activated alongside the sympathetic nervous system, but in the evening, only the sympathetic nervous system was activated. It appears that since the body releases less of the body’s stress hormones in the evening versus the morning, we are more vulnerable to stress in the evening.
Yujiro Yamanaka concluded, "Our study suggests a possible vulnerability to stress in the evening. However, it is important to take into account each individual's unique biological clock and the time of day when assessing the response to stressors and preventing them."
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