We have all experienced stress, some days more than others, but did you know that you may be more vulnerable to stress at certain times of the day? A recent study conducted by medical physiologist, Dr. Yujiro Yamanaka and colleagues, found that the body’s central nervous system reacts less to physiological stress and releases fewer stress hormones in the evening when compared to the morning.
For this study, 27 young and healthy volunteers were recruited. To ensure that no one was at an advantage/disadvantage, all volunteers had normal working hours and sleep habits. Dr. Yujiro Yamanaka and his colleagues investigated the participant’s hypothalamic-pituitary-adrenal (HPA) axis; a subsystem in the body that describes the interactions between the hypothalamus, pituitary glands and adrenal glands. It is known for its role in the body’s reaction to stress.
A major stress hormone in humans is cortisol and it is released from the adrenal glands for several hours once the HPA axis is activated by a stressful event. This hormone provides the body with energy when faced with the need for fight or flight. Essentially, cortisol helps you deal with stressful situations and the brain’s circadian clock regulates cortisol levels.
To determine the baseline of the volunteer’s cortisol levels, researchers measured the participant’s diurnal rhythm of salivary cortisol levels. Diurnal rhythm is a biological rhythm that is in sync with a day/night cycle. Once the baselines were determined, the 27 volunteers were categorized into two groups:
Group 1: Stress test in the morning, 2 hours after participant’s normal waking time
Group 2: Stress test in the evening, 10 hours after participant’s normal waking time
The stress test was 15 minutes long and it involved preparing and delivering a presentation to 3 trained interviewers as well as performing mental math, all while being recorded. To measure cortisol levels, saliva samples were taken at 3 different times:
Half an hour before the stress test
Immediately after the stress test
10-minute intervals for another half hour after the test
This research study found that participants who took the stress test in the morning had higher salivary cortisol levels, whereas those who took the stress test in the evening did not. While the salivary cortisol levels differed between the two groups, their heart rates (sympathetic nervous system indicator that responds to stress) weren’t affected by the period of time that the test was taken.
Overall, researchers concluded that when a stressful situation takes place in the morning, the body can activate the HPA axis and sympathetic nervous system to help manage the stressful event. However, a stressful event taken place in the evening would only activate a response from the sympathetic nervous system (i.e. heart rate). While factors such as an individual’s biological clock needs to be taken into account, there is strong evidence suggesting that stress vulnerability is higher in the evening when compared to the morning.