A new Japanese study finds that the body’s central nervous system reacts less strongly — or releases fewer stress hormones — in response to acute psychological stress in the evening compared to the morning.

The findings, published in the journal Neuropsychopharmacology Reports, suggest we may be more vulnerable to stressful events in the evening.

For the study, medical physiologist Dr. Yujiro Yamanaka and his colleagues from Hokkaido University recruited 27 young, healthy volunteers with normal work hours and sleep habits to investigate whether the “hypothalamic -pituitary-adrenal” (HPA) axis — which connects the central nervous and endocrine systems of the body — responds differently to acute psychological stress according to the time of day.

Cortisol, the primary stress hormone in humans, is released for several hours when the HPA axis is activated by a stressful event. This helps provide the body with energy in the face of a perceived need for fight or flight. Cortisol levels are also regulated by a master circadian clock in the brain, and are normally high in the morning and low in the evening.

In the study, the researchers first measured the diurnal rhythm of salivary cortisol levels from the participants to determine a baseline. The volunteers were then divided into two groups: one that was exposed to a stress test in the morning, two hours after their normal waking time, and another that was exposed to a stress test in the evening, ten hours after their normal waking time.

The 15-minute test involved preparing and giving a presentation in front of three trained interviewers and a camera, as well as conducting mental arithmetic. Saliva samples were taken half an hour before the test, immediately after, and at ten-minute intervals for another half hour.

The results show that salivary cortisol levels increased significantly in the volunteers that took the stress test in the morning while no such response was observed in those that took the test in the evening. However, the participants’ heart rates, an indicator of the sympathetic nervous system which immediately responds to stress, did not differ according to when the test was taken.

“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,” said Yamanaka.

“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.”

Source: Hokkaido University