Researchers from Ben-Gurion University of the Negev discovered that the time of day (day or night) in which you are exposed to a traumatic event can affect the risk of developing post-traumatic syndrome
The time of exposure to the traumatic event during the daily cycle (day or night) was found to influence the behavioral responses and the frequency of developing a post-traumatic syndrome in a rat model.
In a series of experiments conducted by Shlomi Cohen, a doctoral student from the Department of Psychology at Ben-Gurion University of the Negev, under the guidance of Prof. Hagit Cohen, head of the research unit for stress and anxiety research at the Faculty of Health Sciences, it was found that rats exposed to a traumatic event in the early hours of the morning reacted in an adaptive manner and showed less post-traumatic anxiety symptoms -Trauma compared to rats that were exposed to a traumatic event in the evening, in my stages The initial sleep.
The physiological response to stressful events is characterized by the secretion of important hormones such as corticosteroids (cortisol in humans and corticosterone in rodents), which allow the organism to prepare, react and deal with the stress factors. Normal secretion of corticosterone following exposure to a traumatic event, which is proportional to the intensity of the stress, enables a normal response and is one of the most important mechanisms for the individual's survival (Fight or flight responses). Disturbed secretion of this hormone following exposure to trauma was found to inhibit recovery and even increase the risk of developing anxiety, depression and post-traumatic disorders. It is known that the secretion of corticosterone changes in a daily cycle, when in the morning upon awakening the level of corticosterone increases and in the evening, before and at the beginning of sleep, its level decreases. Since corticosterone levels affect the response to the traumatic event, in this study the behavioral responses to exposure to trauma in the morning hours (when corticosterone levels are high) were compared to exposure to trauma at night (when corticosterone levels are low).
In an article recently published in the prestigious journal Neuropsychopharmacology, the researchers showed that the time of day the rats were exposed to trauma affected the behavioral responses measured 7 days after the exposure: rats exposed to trauma in the evening, at the beginning of the resting phase, demonstrated a higher frequency of anxious behavioral responses similar to the post-traumatic syndrome. In contrast, rats that were exposed in the morning, at the beginning of the activity phase, showed responses on resilience and resistance.
To the surprise of the researchers, even though the basic hormone level is higher in the morning hours compared to the evening hours, when measuring the hormone levels in response to trauma, no difference was found in the strength of the response (peak secretion). On the other hand, it will be found that the rate of decline of the hormone is slower in the early morning hours compared to the evening hours. That is, in response to exposure to trauma in the morning, the corticosterone level will remain high for longer, compared to exposure in the evening.
The researchers found that the expression level of a protein known as "neuropeptide Y" changes in the hypothalamus area in a daily cycle similar to corticosterone. This daily variation is related to and affects the corticosterone levels and together they affect the set of long-term reactions to trauma. High expression levels of the peptide are associated with resistant reactions and resilience (in the morning) and its low levels are associated with proneness to anxiety, depression and post-traumatic disorders.
Disrupting the daily cycle of this protein by injecting a specific blocker into the hypothalamus in the morning, canceled the adaptive effect to the trauma and increased the vulnerability. Conversely, injecting the protein itself into the hypothalamus in the evening, caused adaptive responses and increased resilience responses.
These results demonstrate that the daily cycle in the expression levels of both corticosterone and neuropeptide Y, as well as the mutual relationships between them, influence the response to exposure to a traumatic event. In the morning hours, at the peak of their excretion, the response to exposure to trauma will be adaptive and accompanied by a low risk of developing post-traumatic stress, while in the evening hours, when their level is low, the response to exposure will be accompanied by a higher risk of mental pathology.
The researchers believe that the time of day when the trauma occurs has a significant effect on the set of long-term mental reactions. These findings are of great importance not only in understanding the physiological-molecular mechanism involved in the response to exposure to trauma, but in defining the time of exposure as a risk factor for vulnerability following a traumatic event.
