To little sodium can cause Hyponatremia which can lead to nausea, muscle cramps, disorientation, slurred speech, confusion, and inappropriate behavior. Also alot of those symptoms are linked with dehydration and if you drink more water it will increase the problem and in extreme cases seizures comas and death can happen.
About 500mg of sodium a day is considered the least you need. That's 1/4 teaspoon of table salt.