Anxiety events in the evening release less of body stress hormones than those happening in the morning, according to a new study by the Japanese University of Hokkaido.
In the study, which has been published in the journal Reports of Neuropsychopharmacology, physicist Yujiro Yamanaka and his colleagues recruited 27 young, healthy volunteers with normal working hours and sleep patterns to see if the HPA responds differently to acute psychological stress depending on the time of day.
The HPA axis connects the central nervous and endocrine system of the body. Cortisol, the primary stress hormone in humans, is released for several hours when the HPA axis is triggered by an anxiety event. This helps to provide the body with energy against a perceived need for a race or flight. Cortisol levels are also regulated by a main circadian clock in the brain and are usually high in the morning and low in the evening.
The group first measured cortisol levels in the volunteer saliva to determine a baseline. The volunteers were then divided into two groups: one exposed to a stress test in the morning, two hours after the normal wake-up time, and another exposed to a stress test at night, ten hours after the normal wake-up time.
The test lasted for 15 minutes and included preparation and presentation in front of three trained transponders and a camera and conducting a mental arithmetic. Saliva samples were taken half an hour before the start of the test, immediately after, and at ten minute intervals for another half hour.
The researchers found that salivone cortisol levels increased significantly in volunteers who took the stress test in the morning, and no such response was observed in those who took the test at night. The heart rate of the volunteers, on the other hand, an indicator of the sympathetic nervous system that responds directly to stress did not differ depending on when the examination was taken.
Principal study author Yujiro Yamanaka said: "The body can respond to morning stress by activating the HPA axis and the sympathetic nervous system, but it must respond to night-time stress by only activating the sympathetic nervous system. Our study shows potential sensitivity to evening anxiety. However, it is important to take into account the individual biological clock of each individual and the time of day when assessing the response to and the prevention of stressors. "