Yujiro Yamanaka, Ph.D. Associate Professor Hokkaido University Graduate School of Education Sapporo, Japan 

Stress at Night May Be More Stressful

MedicalResearch.com Interview with:

Yujiro Yamanaka, Ph.D. Associate Professor Hokkaido University Graduate School of Education Sapporo, Japan 

Dr. Yamanaka

Yujiro Yamanaka, Ph.D.
Associate Professor Hokkaido University
Graduate School of Education
Sapporo, Japan 

MedicalResearch.com: What is the background for this study?

Response: My laboratory has focused on the human circadian rhythms in particularly investigating the time of day effect of non-photic cues especially physical exercise and psychological stressor on circadian rhythms.

In mammals including humans, the central circadian pacemaker is located in the suprachiasmatic nucleus (SCN) in the brain hypothalamus. The SCN entrains to an external light-dark cycle and generates endogenous 24 h rhythmicity in body function. The hypothalamic-pituitary-adrenal (HPA) axis is our major stress response system. We can assess the response of HPA axis to psychological stress event by measuring the level of glucocorticoid hormone, cortisol in saliva. The cortisol shows clear circadian rhythm with higher levels in the morning and lower levels in the evening. This rhythm is generated by the SCN circadian pacemaker.

Previous study (Kudielka et al. Psychoneuroendocrinology 2004) could demonstrate that the cortisol stress level is significantly elevated by acute psychological stress event in the morning (9 a.m. to 10 a.m.) and afternoon (3 p.m. to 4 p.m.). However, there are no study examining the effect of evening psychological stress event on the HPA axis activity. Thus, our new study focused on examining whether the HPA axis differentially responses to morning and evening stress event in healthy subjects.

MedicalResearch.com: What are the main findings? 

Response: The main finding in this study is that the HPA axis activity response to acute psychological stress is higher in the morning than in the evening. We could find that salivary cortisol levels increased significantly by the stress test in the morning while no such response was observed in the evening. In contrast to the time of day difference in cortisol stress response, we could not find any significant difference in the heart rate both in the morning and evening. The heart rate significantly increased just after the stress event. The heart rate is a marker of sympathetic nervous activity. In other words, our body must response to evening psychological stress event by activating only the sympathetic nervous system. 

MedicalResearch.com: What should readers take away from your report?

Response: Our body can respond to the morning stress event by activating the HPA axis and sympathetic nervous system, but it needs to respond to evening stress event by activating the sympathetic nervous system only.

Our study suggests a possible vulnerability to stress in the evening. Please beware of evening stress. 

MedicalResearch.com: What recommendations do you have for future research as a result of this work?

 Response: In the current modern society, it has been known that many people engage in night work and rotating night shifts. Long term night works may face on stress event for workers and increase risk of various health problem. Future studies need to examine the relationship between the HPA axis stress response at mid-night and health problem in people engaged long term night work. 

Yujiro Yamanaka, Hidemasa Motoshima, Kenji Uchida. Hypothalamic-pituitary-adrenal axis differentially responses to morning and evening psychological stress in healthy subjects. Neuropsychopharmacology Reports, 2018; DOI: 1002/npr2.12042 

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Last Updated on November 29, 2018 by Marie Benz MD FAAD