Smell is an important primal sense. The first major nerve that enters the brain, the olfactory nerve, detects scent. That nerve, and the information it carries, connects directly to the base of the brain, where immediate responses occur.  

In his best-selling book, Perfect Health, Dr. Deepak Chopra explains why aromatherapy works:

The language of taste is limited to sweet, sour, salty, bitter, astringent, and pungent. The nose, on the other hand, understands a vast vocabulary of smells, amounting to about ten thousand different odors if you have a well-trained beak. The odors that can be detected by the nose must first dissolve in the moisture of the nasal tissue and are then passed on by specialized olfactory cells straight to the hypothalamus in the brain . . . 

The fact that smells go straight to the hypothalamus is very significant, for this tiny organ is responsible for regulating dozens of bodily functions, including temperature, thirst, hunger, blood sugar levels, growth, sleeping, waking, sexual arousal, and emotions such as anger and happiness. To smell anything is to send an immediate message to “the brain’s brain” and from it to the whole body. At the same time, the message of an odor goes to the brain’s limbic system, which processes emotions, and to an area called the hippocampus, the part of the brain responsible for memory, which is why smells bring back past memories so vividly.

So, when you are applying essential oils topically to assist mood, it is important to get those oils as close to the limbic system as possible and apply them where they can best bypass the blood-brain barrier. Directly below the base of the skull, on both sides of the neck, you can feel a place of indention. This area is called the suboccipital triangle, and when pure essential oils are applied there, they can enter the circulatory system of the brain before entering the circulatory system of the body. You can also apply them to the mastoid bones behind the ears, across the front of the forehead, directly under the nose, and even to the roof of the mouth (place oil on the pad of your thumb and then place on the roof of your mouth) for more direct access to the limbic system.