An article from Mark's Daily Apple:
How Long Does It Take for Fitness Benefits to Show?
So when can we expect some benefits from our hard work?
Some benefits “show” immediately as long as you have the right tools to test them. If you were to take a muscle biopsy, you’d see that muscle protein synthesis—the incorporation of protein into muscle fibers, the first signs of muscular hypertrophy—begins just four hours after you lift something heavy.
If you were to sample some bone marrow after a single bout of resistance training, you’d notice that you just upregulated production of the cells that heal your endothelial lining.
If you were to hold a stethoscope up to your hamstrings immediately after a sprint session, you’d hear the *pop-pop-pop* of new mitochondria being spawned.
But those aren’t visible to the naked eye, nor are they accessible to the masses. We don’t feel our bone marrow pumping out progenitor cells. Amazon doesn’t sell muscle biopsy kits.
What benefits can we expect (and notice) immediately?
Fat oxidation: Many types of execise will boost the amount of fat you burn over the course of a day. Sprinting, high-intensity intervals, and even doing moderate aerobic activity before breakfast all increase 24-hour fat oxidation.
Insulin sensitivity: A single bout of intense exercise—lifting, running, sprinting, CrossFit—will deplete glycogen and improve your insulin sensitivity. You can’t “feel” that, but you will notice the improved postprandial blood sugar, as well as the ability to consume more carbohydrates without gaining weight the next day.
Endorphins: One immediate benefit is the release of endogenous opioids, also known as beta-endorphins. Studies indicate that it takes about an hour of endurance training for beta-endorphins to release (the runner’s high), while short-term anaerobic training produces similar effects in even less time. One study found that an acute bout of low-volume, high-intensity Olympic weightlifting caused elevations in beta-endorphin. High-intensity anaerobic work produces the biggest endorphin rush for your buck.
Neural strength: Improvements to neuromuscular efficiency happen within days of starting a lifting program. Beginners get stronger almost immediately simply by learning proper technique and how to fully contract the muscle fibers they already have.
Mood: Acute exercise boosts mood immediately after training, even in patients with major depressive disorder.
Sleep: Exercising in the day has a modest but beneficial effect on sleep that night.
Cognitive function: A single bout of high-intensity interval training boosts cognitive performance and brain-derived neurotrophic factor.
Gene expression: A single bout of endurance training also triggers gene expression in both exercising and non-exercising muscles, upregulating the genes responsible for fat oxidation and metabolism.