Other than in DNA, nucleotides can also act as messengers or modulators in the body. For example, the nucleotide adenosine (adenine plus ribose without the phosphate) may be the most important type of immunomodulator (a compound that increases or decreases neuron, or nerve, activity). In fact, adenosine is used clinically to stop a person’s heart when it is beating erratically; the natural heart pacemaker cells then return the heart to its normal rhythm. Adenosine is also thought to play a role when you feel fatigued or tired; in fact, neuroscientists currently theorize that caffeine works to maintain alertness by interfering with the reception of adenosine on a cell’s surface.