Japanese researchers have discovered that stressful events in the evening tend to release less of the body’s stress hormones than an equivalent event that takes place in the morning. The research suggests a greater vulnerability to stress in the evening. The implications of this are not only for general stress management, they also extend to times of working, such as nightshift activities.
Most animals have adapted to a day-night cycle by evolution of internal circadian clocks. This mechanism adjusts behaviour and physiology to the recurring changes in environmental conditions. One function of the internal body clock is with the regulation of the stress response.
To examine the impact of stressful events at night compared with the day, the researchers recruited 27 volunteers, pre-assessed of being of average health. The volunteers worked standard 9-to-5 hours and had good sleep patterns. The researchers aimed to see whether the hypothalamic -pituitary-adrenal axis responds differently to acute psychological stress according to different times of the day. The axis connects together the central nervous and endocrine systems of the body.
It has previously been established that cortisol, the main stress hormone in humans, is released for several hours when the axis is activated during a stressful event. The release of this hormone assists the body with energy during a stressful situation. Cortisol levels are regulated by the circadian clock. These levels are high in the morning and low in the evening.
The researchers took samples of saliva and established the rhythm of salivary cortisol levels in the volunteer subjects. One group of volunteers were exposed to a stress event during the morning and the second groups were exposed to a stress event during the evening (at a time point after the subjects had been awake for ten hours). The stress event was running a presentation in front of a camera.
While the heart rates of the volunteers were similar, the cortisol levels varied considerably between day (higher) and night-time (lower). The data, assessed by researchers who were from Hokkaido University, suggests that human body’s central system reacts less strongly to acute psychological stress when occurs in the evening compared with the morning. These findings have implications for health and wellbeing.
The findings have been published in the journal Neuropsychopharmacology Reports. The research is titled “Hypothalamic-pituitary-adrenal axis differentially responses to morning and evening psychological stress in healthy subjects.”
