As a 3-4l of water/day type of person I have first hand encountered the lack of salt (cramps while I sleep) and also the solution: add salt to your diet. I makes the food taste nice to boot!
That's pretty rare in the modern world - most people chronically consume too much salt.
This recent (Apr 2017) interview[0] with Prof. Graham MacGregor[1] is really interesting on the topic of salt consumption. He says most people need add no extra salt to their food, relying instead on the small, naturally occurring amounts in the things we eat. I've done exactly this and noticed no ill effects (cramps, etc.).
oooh, I wonder if that's why I sometimes get cramps while I sleep. I feel like if I stretch my legs too much while in bed, I could randomly get a cramp.