When prolonged, excessive training stresses are applied concurrent with inadequate recovery, performance decrements and chronic maladaptations occur. Known as the overtraining syndrome (OTS), this complex condition afflicts a large percentage of athletes at least once during their careers. There is no objective biomarker for OTS and the underlying mechanism is unknown."
Source : The Unknown Mechanism of the Overtraining Syndrome in: Sports Medicine, March 2002, Volume 32, Issue 3, pp 185–209
There are estimates that up to 10% of elite endurance athletes are sometimes affected by overtraining syndrome - which is defined as 'an unexplained underperformance for approximately 2 weeks even after having adequate resting time'.
Symptoms can include :
See : Wikipedia
Also see Exercise addictionplugin-autotooltip__plain plugin-autotooltip_bigExercise addiction
'Addiction' to exercising is currently classed as a 'behavioural addiction' whereby a person becomes 'hooked' on the euphoria and pleasure that it provides (to some individuals).
It's known that heavy exercise can generate the release of hormones known as