27 healthy young volunteers were recruited with normal work hours and sleep habits to investigate whether the hypothalamic pituitary adrenal axis responds differently to acute psychological stress according to the time of day.
Before the study to establish baselines each of the subject’s diurnal rhythm of salivary cortisol levels were measured, who were then divided into two groups: one being exposed to morning stress after 2 hours from their normal waking time; and the other being exposed to evening stress ten hours after their normal waking time. Stress testing lasted for 15 minutes and involved preparing and giving a presentation in front of 3 interviewers and a camera, and conducting mental arithmetic. Saliva samples were taken 30 minutes before the test, immediately after, and then a 10 minute intervals for another 30 minutes.
Salivary cortisol levels were found to have significantly increased in the subjects who took stress tests in the morning, those who took evening stress test did not have that response. Heart rates of all of the subjects did not differ according to the time of when the test was taken.
The body responds to morning stress by activating the HPA axis and sympathetic nervous system, and responds to evening stress events by activating the sympathetic nervous system only. According to the researchers their findings suggest a vulnerability to evening stress, but note it is important to take into account each person’s unique biological clock when assessing response to stressors and preventing them.
