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).
Though the researchers expected to find a correlation between the behavioral response and the corticosterone levels in the bloodstream, to their surprise they instead found that although the basal levels of corticosterone were significantly different between the morning and night, the magnitude of the HPA axis response to stress was not statistically different between the times.
“However, a slower return to baseline corticosterone levels after stress exposure was observed in the morning,” Prof. Cohen reported. “In other words, during stress exposure in the morning, high corticosterone levels are prolonged in comparison to stress exposure at night.”
These findings imply an association between the circadian phase, HPA regulation and the behavioral response to stress.
Rats exposed to the stressor during the night displayed faster post-exposure corticosterone decay and a more pronounced stress-induced decline in neuropeptide Y (NPY) expression in the hypothalamus.
Blocking hypothalamic NPY prior to stress applied in the morning, or administering NPY to the hypothalamus prior to stress applied in the night, had a resounding behavioral effect.
The authors thus suggest that the daytime variations in HPA and NPY significantly affect the behavioral response, so that we are more resilient at the beginning of the morning and more vulnerable at the beginning of the night.
The takeaway message? While we cannot always control at what times of day we will experience a stressful event, it may be wise to put off a stressful encounter, or the announcement of bad news, till the morning when we are able to cope better.