The body's response to stress is similar no matter what the cause. The short-term reaction is the "fight-or-flight" response, consisting of activation of the sympathetic nervous system and the release of adrenaline. Longer-term stress activates the HPA axis, which causes cortisol to be released from the outer adrenal cortex. Cortisol has similar effects to adrenaline in the body, but remains in blood for a longer period of time. The release of cortisol is ultimately controlled by the body clock, and is released in a circadian pattern with the highest levels around 8-10am in the morning and lowest levels around midnight. During periods of stress, the neurons in the hypothalamus which control the amount of cortisol released increase their output, resulting in a higher overall circadian release of cortisol.
Patients suffering from CFS and burnout show reduced or suppressed HPA axis, and lower than normal levels of cortisol, as well as a less pronounced circadian rhythm of cortisol and other hormones. In the past very little research was done into this state of low cortisol, and it was simply ignored. New research, however, shows that this state of low cortisol or burnout is caused by a number of factors, with mental attitude and lifestyle being the most important.
What seems to happen is that the body reduced its HPA axis activation when it appears that participating in further stressful activities may not be beneficial. This results in a reduced ability to handle stress (reduced "stress tolerance"), and in extreme cases results in illnesses such as CFS.
As well as providing the body with energy and vitality, the HPA axis also activates the immune system, so a reduced HPA axis activation results in poorer immunity to illness as well as a reduced tolerance to physical and mental stresses.
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