Caffeine and guarana are both stimulants that affect the central nervous system. <a href="https://awareofcare.com/how-much-caffeine-in-vitamin-water-focus/">how much caffeine is in vitamin water focus</a>  They work by blocking the action of adenosine, a neurotransmitter that promotes sleep and relaxation. By blocking adenosine, caffeine and guarana increase alertness and energy levels.