What we perceive as 'energy' is the cellular capacity to produce ATP — the energy currency that sustains every function of the body. ATP production takes place in the mitochondria and depends on specific cofactors: B-group vitamins (B1, B2, B3, B5, B6, B12) in active forms, magnesium in bioavailable forms, iron and complementary micronutrients.
Subclinical fatigue —the kind that does not reach a formal clinical category but impairs quality of life— often responds to suboptimal deficiencies of these cofactors. It also responds to dysregulation of the hypothalamic-pituitary-adrenal axis caused by chronic stress, which adaptogens such as cordyceps and rhodiola can modulate without acting as stimulants.
The difference between supplementing energy cofactors and nervous stimulation matters. Caffeine and other stimulants generate a perception of energy via the central nervous system, without increasing actual ATP production. Cofactors act on the cellular processes themselves. They serve different functions and do not replace one another.