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"Our study suggests a possible vulnerability to stress in the evening," said study leader Yujiro Yamanaka, a medical physiologist at Hokkaido University in Sapporo.
"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," Yamanaka said in a university news release.
The researchers measured levels of the stress hormone cortisol in the saliva of 27 young, healthy volunteers.
They also exposed them to a stressful situation to see if the hypothalamic-pituitary-adrenal (HPA) axis responds differently to acute stress at different times of day.
The HPA axis connects the body's central nervous and endocrine systems. When a stressful event activates the axis, the body releases cortisol.
Cortisol levels are also controlled by the circadian clock in the brain.
The volunteers were divided into two groups. One group was exposed to a stress test in the morning, two hours after waking up. The other group was exposed to a stress test in the evening, after being awake for 10 hours.
The 15-minute test involved preparing and giving a presentation to three interviewers and a camera. Participants also had to do mental arithmetic.
The researchers took saliva samples half an hour before the test, immediately after, and at 10-minute intervals for another half hour.
Cortisol levels increased significantly in the morning, but not in the evening, the results showed. However, participants' heart rates -- an indicator of the sympathetic nervous system's response to stress -- did not differ.
"The body can respond to the morning stress event by activating the HPA axis and sympathetic nervous system, but it needs to respond to evening stress event by activating the sympathetic nervous system only," Yamanaka said.
The report was published recently in the journal Neuropsychopharmacology Reports.
-- Steven Reinberg
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