Peppermint: "Peppermint purifies and stimulates the mind. It also can increase mental alertness,” Scarpelli advised. "Dr. William N. Denver of the University of Cinncinati found that inhaling peppermint oil increased mental accuracy by 28 percent.” Peppermint oil is also one of just a handful of oils that can be taken internally, which is fortunate because it’s a great recipe to help indigestion — just mix a few drops in water and drink.