Sufficient fluid intake can elevate alertness and reduce brain fog by activating the sympathetic nervous system.

“We know that there are two mechanisms by which fluid intake triggers elevated levels of alertness, and it all has to do with the so-called sympathetic arm of the autonomic nervous system... when we have fluid in our gut and when our cells are well hydrated, and believe it or not, when our bladder contains fluid within it, there is an elevation in activity of the sympathetic nervous system by way of two pathways. One is mechanical. In fact, we have so-called stretch receptors in our bladder and in our gut, these stretch receptors have fancy names like TRP trip channels as they're called, or piezo, which are these stretch sensing channels.”