 Caffeine's mechanism of action is somewhat different from that of cocaine and the substituted amphetamines; caffeine blocks adenosine receptors A1 and A2A.Adenosine is a by-product of cellular activity, and stimulation of adenosine receptors produces feelings of tiredness and the need to sleep.Caffeine's ability to block these receptors means the levels of the body's natural stimulants, dopamine and norepinephrine, continue at higher levels.