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/.