Hyponatremia
Hyponatremia is a condition known as "water intoxication." It is the opposite of dehydration, and is often associated with long distance events like running and cycling. Moreover, it’s not an unusual problem, and you can develop it in a few hours.
As you consume large amounts of water over the course of a day, blood plasma (the liquid part of blood) increases thereby diluting the salt content of the blood. At the same time, your body also loses salt by sweating. Consequently, the amount of electrolytes available to your body tissues decreases over time to a point where that loss interferes with brain, heart, and muscle function! You have to replace these electrolytes! They're essential to the normal electro-chemical operation of your nervous system.
Symptoms
Though the condition is opposite to dehydration, the symptoms generally mirror those of dehydration (apathy, confusion, nausea, and fatigue), although some individuals show no symptoms at all.
If untreated, hyponatremia can lead to coma and even death.