roadhouse said:nothing happens overnight and nothing is immediate.
Several decades ago, a study from John Holloszy's lab showed that the activity of delta amino levilnate synthase, the rate-limiting enzyme in heme biosynthesis and hence increases in components of the electron transport chain in mitochondria, increased within 18 h a single bout of exercise.
More recent studies examining regulators of mitochondrial biogenesis such as PGC1-alpha have shown increases w/in a few hours of an acute bout of exercise.
Undoubtly due to the rapidity of such adaptations, the half-life for changes in things such blood lactate concentration or RER with changes in training load is about 6-10 d.
So, yes, things do, in fact, happen "overnight"...