The chemical adenosine is released by brain cells when those cells are active. Adenosine then binds to more and more sites on cells in certain areas of the brain, as the total amount released gradually increases during wakefulness. During sleep, the number of sites to which adenosine is bound decreases. Some researchers have hypothesized that it is the cumulative binding of adenosine to a large number of sites that causes the onset of sleep.
The statement 'Caffeine, which has the effect of making people remain wakeful, is known to interfere with the binding of adenosine to sites on brain cells' provides the most support for the researchers' hypothesis.