Caffeine affects the human body at a cellular level primarily by acting as a stimulant to the central nervous system. It blocks the action of adenosine, a neurotransmitter that promotes sleep and relaxation, which leads to increased alertness and reduced fatigue. This blockade causes the release of other neurotransmitters, such as dopamine and norepinephrine, enhancing mood and cognitive function. Caffeine can also influence the metabolic rate by stimulating fat oxidation and increasing energy expenditure, making it a widely consumed substance for improved mental and physical performance.