I'm always surprised when people don't know how important salt intake is, your body needs salt to use water. Sweat, tears, spit and pee all contain salt. Our bodies without a large intestine which does most of the absorbing of these materials needs extra help. Our bodies actually use salt in many ways.
My doctor's explained this before I even had the jpouch surgery. After surgery, they were happy enough to hear that I was mixing salt in my water jug for work, and it definitely makes a difference when I am sweating a lot.
Lack of salt, potassium and dehydration all cause lethargy and are worse when experiencing more than one.
Good luck, stay hydrated, salted and eat some potatoes (or have some potassium).