Sodium is one of the electrolytes, the minerals with an electric charge which leap around through blood and extracellular fluid between them all hydrating (rather than drying out) the body, regulating blood pressure, keeping blood pH (acidity level) right, allowing tissues to heal, and tons, tons more(1). Human blood is about .9% salt - muscles cramp and weaken without it plus exhaustion sets in(2) - especially important to address in athletic activities where electrolytes sweat out all the more(1). Astronauts who ate more salt actually reported drinking less water yet urinating more(2). 
   There is salt, and salt, though. Table salt processes and bakes at close to 1,200*F to where it loses at least about 80 beneficial natural minerals and nutrients, strips down essentially to sodium chloride, and otherwise plain chemically alters in the process. Yet, some addictive substances like neurotoxic aluminum get added in in turn. This also includes generic refined sea salt if not labeled as unrefined(2). [In other words, all considered, low-sodium diets tend to censure all salt versus just nasty table salt]
   A half a teaspoon of raw, unrefined salt, or 3 grams, is considered a rule of thumb for bodily hydration per 10 glasses of water(2). My parents and I prefer the taste of Himalayan pink salt (white Himalayan is refined). We take a pinch at a time during the day if our lips are dry.

Sources:
1.  "Why Electrolytes Are So Important." Isagenix Health, Isagenix Worldwide, Inc., 16 February 2015, https://www.isagenixhealth.net/why-electrolytes-are-so-important/.
2.  "The Right Salt Is Essential | Salt Gets A Bad Rap." Colorful Canary, 25 February 2010, https://www.colorfulcanary.com/2010/02/salts-gets-bad-rap.html.

   They perch atop the kidneys. Think the "fight-or-flight" response - cortisol raises blood sugar(1). Adrenaline(1,2) gets the heart pumping and blood to the brain and muscles.  Add in the ability to regulate salt to water ratios and blood pressure via hormones(2). They are the crucial adrenal glands!
   The adrenals (A) relate with the hypothalamus (H) and pituitary gland (P) to where they constitute the "HPA axis." The hypothalamus is an organ signaler. It signals the pituitary gland to make growth hormone(1). The "H" and "P" together call on the adrenals to make other hormones for blood sugar; blood pressure; immune responses; inflammatory reactions; and the conversion of fats, proteins, and carbs into energy(2).
   Adrenal fatigue has a more technical term, "HPA Axis Dysfunction." Why? Cortisol is ordered by the hypothalamus and pituitary gland, then when turned out in sufficient amounts normally forms an example of a "negative feedback loop" - it turns off the very signals from the "H" and "P" calling for it. But, with adrenal fatigue, the "H" and "P" become so overburdening with relentless signals that the adrenals just give out and can't fill out further orders! In turn, this exhausts energy levels, mind, memory, bone mass, muscles, immune system, and even the ability for sleep to ready the body for the day to come(1).
   Besides these "exhaustions," adrenal fatigue also beefs up food sensitivities, joint pain, and anxiety at day or night(1). A tendency for adrenal fatigue sufferers is to be tired through the day, commonly with caffeine and other stimulant dependency. Some may in severe cases stay awake only a few hours at a time. Some will suddenly get an energy rush late at night and finally ride it out in the wee morning(2).
   Some exhaust their adrenals with lifestyles with loads of caffeine, nicotine, poor sleep, and chronic stress - candida will eventually lead to adrenal fatigue and stimulants like caffeine and toxic gut health will promote it further. Heavy metal toxicity, thyroid issues, and infections (especially respiratory) will also exhaust the adrenals. Addison's disease and adrenal cancer also affect how the adrenal glands operate(2).

Sources:

1. Luther, Daisy. "5 Things You Should Know About Adrenal Fatigue."The Organic Prepper, 18 July 2017, https://www.theorganicprepper.com/adrenal-fatigue/.

2. "Adrenal Fatigue." TruthWiki: Get the Real Story. Approximately 4 years ago. http://www.truthwiki.org/adrenal_fatigue/.