Adenosine receptors are proteins located on the surface of cells that bind to adenosine, a naturally occurring molecule that accumulates in the brain during waking hours and promotes the drive to sleep.

They are found throughout the body, with particularly high concentrations in the brain, heart, and immune tissue, where they regulate a range of physiological processes including neuronal activity, cardiovascular function, and inflammation.

When adenosine binds to its receptors in the brain, it inhibits neural activity and produces feelings of drowsiness and reduced alertness.

Caffeine exerts its stimulating effect primarily by blocking adenosine receptors, preventing adenosine from binding and thereby suppressing the sleep drive signal.