Guarana

Guarana is a climbing plant from the Amazon whose seeds are rich in caffeine, a stimulant that blocks adenosine, the brain chemical that makes you feel drowsy. People mostly look at it for fatigue, but the evidence is very limited: a small study of 20 people found no clear difference in cognition. With so few participants, it is too early to say much either way, and the results may not carry over to everyone.