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Scientific Overview
Stress and Stress Tolerance
HPA Axis
Circadian Rhythms
Immune System
Placebo Effect
Burnout
Chronic Fatigue Syndrome
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Irritable Bowel Syndrome
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Circadian Rhythms

Many of the hormones in the body maintain a roughly 24-hour rhythm, controlled by the suprachiasmatic nucleus (SCN)—also known as the body clock—in the hypothalamus. The HPA axis — the body's stress/energy regulator — is one of the most important circadian rhythms in the body. As well as regulating cortisol levels, it has such a great influence over health and vitality, being an important modulator of the immune system, energy regulation, the digestive system and reproductive system.

Figure: Typical variation in cortisol levels throughout the day in a healthy individual

In healthy individuals, cortisol has a diurnal variation, rising in the 30-60 minutes after awakening, then dipping, followed by another peak in the late afternoon.

Different systems in the body have their own body clocks, such as the digestive system and core body temperature. These circadian rhythms are reset by the "master" body clock (the SCN).

Abnormal hormone rhythms have been implicated in chronic fatigue syndrome (CFS), seasonal affective disorder (SAD) and insomnia. Research shows that CFS patients tend to have a less pronounced (more flat) circadian rhythm of HPA axis activation, and that the patients who recover most tend to have the greatest increase in circadian hormone variation (peak to trough).

Research also shows that high levels of exercise causes the circadian rhythm to shift in CFS patients, causing their sleep-wake cycle to get later, but not in controls.

Having a circadian rhythm rather than a constant level of certain hormones, such as cortisol and melatonin, is important to many systems in the body, and the abolition of the normal rhythm is likely to cause many of the symptoms associated with illnesses such as CFS. A dysregulated body clock significantly affects sleep, and insomnia has been shown to exacerbate CFS symptoms.


References and external links

Papadopoulos, Andrew S., and Anthony J. Cleare. "Hypothalamic-pituitary-adrenal axis dysfunction in chronic fatigue syndrome." Nature Reviews Endocrinology 8.1 (2012): 22-32.

Tanaka, M., Tajima, S., Mizuno, K., Ishii, A., Konishi, Y., Miike, T., & Watanabe, Y. (2015). Frontier studies on fatigue, autonomic nerve dysfunction, and sleep-rhythm disorder. The Journal of Physiological Sciences, 65(6), 483-498.

Wikipedia page on circadian rhythm

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