If you have noticed that you are less able to handle stressful situations at night, you may not be surprised by the results of an Israeli experiment showing that the time of day at which stress occurs significantly affects the behavioral response.
Animals are measurably more vulnerable to stress during the night and more resilient in the morning, according to findings by graduate psychology student Shlomi Cohen, working with other scientists under the guidance of Prof. Hagit Cohen, director of the Stress and Anxiety Research Unit in the Faculty of Health Sciences at Ben-Gurion University of the Negev.
Their findings were recently published in the Neuropsychopharmacology Journal.
When we experience stress, our adrenal cortex releases appropriate amounts of hormones such as glucocorticoids (cortisol in humans and corticosterone in rodents) that enable the flight-or-fight response of preparation, response and coping — both physically and emotionally. This process is governed by the hypothalamic–pituitary–adrenal (HPA) axis.
Inadequate corticosterone release following stress not only delays recovery but can also interfere with the processing or interpretation of stressful information, and alters the trajectory of trauma exposure.
Time of day matters
The researchers knew that the HPA axis displays a characteristic circadian pattern of corticosterone release, with higher levels at the beginning of the morning and lower levels at the beginning of the night. For their experiment, they exposed lab rats to stress at these two times and looked for differences in their response.
Assessing the rats’ behaviors seven days afterward, the team saw that the time of day of the traumatic exposure markedly affected the pattern of the behavioral stress response and the prevalence of rats showing an extreme behavioral response (PTSD-like behavioral responses).
Rats exposed to the stressor at the beginning of their inactive phase (night) displayed a more traumatic behavioral response, and, conversely, were more resilient to stress exposure at the beginning of the active phase (morning).