There ae many physiological changes which occur with pain, from increased heart rate, blood pressure and autonomic responses (sweating, respiration increases, etc) to chemical changes such as releases of endorphins (several types), serotonin, adrenalin (epinephrine) and noradrenalin (norepinephrine). This altered biochemistry can mess up a lot of things, including sleep patterns, autonomic function, activity levels. In turn, this can create other biologic changes. You can see that it would be hard to identify which effects are caused by pain exclusively. But that really doesn't matter, because all of these things happen when you get pain, especially for long periods, so you will never see one problem in isolation.
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