Short-term overexertion is rarely harmful.
Chronic overexertion is risky and harmful, though, because it causes chronic overstimulation of your immune and nervous systems, resulting in an increase in (invisible) inflammatory reactions in a variety of organs
micro-inflammation or inflammation of your neurons or nerve cells in the brain increase the risk of depression, Alzheimer's disease, and a whole range of neurological or psychiatric disorders
microinflammation or inflammation of your blood vessels can cause arteriosclerosis or atherosclerosis, which in time can cause diseases such as heart attack, cerebral infarction or CVA, micro-embolismën or ultra-small blood clots in the brain that cause accelerated dementia
This is why it is wise to detect chronic overexertion early via a online self-test and, if necessary, to address it preventively with a professional prevention program.