[Filligree]

Higher sodium = your blood is saltier = insufficient hydration.

Your body attempts to keep it at a constant level. This is much easier if you drink more than you need to; it's easy to remove excess water, harder to remove excess sodium especially if you're about to drink more water and would then be sodium-deficient. But in general, serum sodium shouldn't vary much unless you're at least a little dehydrated.

I try to always keep some water handy, since if I don't, I have a tendency not to go grab it until a bit later than I really should have.