Human bodies which experience stress in the evening release less cortisol -- the primary stress hormone in humans -- compared to stressful events in the morning, and thus may pose vulnerabilities, according to a new research.The study, led by medical physiologist Yujiro Yamanaka at Japan's Hokkaido University, the body's central system reacts less strongly to acute psychological stress in the evening than it does in the morning, suggesting possible vulnerability to stressful events in the evening."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," Yamanaka commented.The study, published in the journal Neuropsychopharmacology Reports, explored a small group of young and healthy volunteers with normal work hours and sleep habits to find out if the "hypothalamic-pituitary-adrenal" (HPA) axis responds differently to acute psychological stress according to the time of day.
The HPA axis connects the central nervous and endocrine systems of the body. Cortisol 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.The team first measured the diurnal rhythm of salivary cortisol levels from the volunteers and then exposed one group to stress test in the morning, and another to the evening.The researchers found 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."Our 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.