Transient mental fogginess can be caused by over thinking, overworking, and fatigue. Essential oils can help to clear the mind, relieve fatigue, and sharpen your mental focus. Rosemary and basil are two of the best essential oils for stimulating brain function. Place a drop or two of either one (or a combination of both) on a handkerchief and inhale the fragrance as needed, or make the following air freshener to spray into the air when you need a boost of mental clarity.