This important chemical is used to transfer energy within all the cells of the body, and it's also one of the chemical messengers, or neurotransmitters, within the brain. It has an essential function in many biochemical processes and serves a vital role in sleep initiation. Adenosine is believed to be responsible for the increasing levels of sleepiness that exist the longer a person stays awake. During wakefulness, adenosine levels gradually increase in areas of the brain that are important for promoting arousal, especially the reticular activating system in the brainstem. With higher and higher concentrations, adenosine inhibits arousal and causes sleepiness. Therefore, high levels of adenosine in effect cause sleep. Interestingly, caffeine works by blocking the action of adenosine within the brain, which increases wakefulness.