Tonalli /to(ː)nalli/ (see also: Tonal) plays a multiplicity of roles; acting as a day sign, body part, and a symbol of the sun’s warmth. Ancient Nahuatl people believed that it was located in the hair and the fontanel area of one’s skull, and that the tonalli provided the “vigor and energy for growth and development”. It often overlaps with the force of teyolía which was often considered both an animating force (soul) and the physical heart in various Mesoamerican cultures.
The root “tona” acts as an adjective to mean "to irradiate or make warm with sun”.
In the Ancient Nahuatl belief, the tonalli is bestowed upon a child in utero by the aged deities known as Ometecuhtli and Omecihuatl, or the “Lord and Lady of Duality”. The implementation of tonalli is conducted through a process known as Fire Drilling.
It is believed that the old deities, Ometecuhtli and Omecihuatl transferred tonalli to human fetuses by “simultaneously breath[ing] the tonalli into the child and ignit[ing] a fire in its chest”. This Fire Drilling process involves an upright wooden piece being twirled rapidly on a flat base. It produces heat through friction, although this seemingly simple instrument requires considerable skill to make anything but smoke. The fire maker blows on an ignited spark to fan it into a vigorous flame, and the breathing (or blowing air) and friction in the chest animate an infant. The Franciscan friars connected this idea of Fire Drilling, namely, the conception of tonalli as breath, to Christianity as the infusion of breath into the body recalls the beginning of Genesis, where God the Father breathes life into Adam.
The Nahuatl people of Mesoamerica believed that the soul comprised three entities: Tonalli, Teyolía, and Ihíyotl, three souls in the body. Tonalli is located in fontanel area of the skull. Teyolía is located in the heart and Ihíyotl is in the liver. Each of these souls has its own functions and protective deities. But there are important differences. The Tonalli is the soul that enters and leaves the body. In Atla in the northern Sierra de Puebla, the inhabitants believe this is the soul that travels while you sleep at night, and then comes back. This is the soul that leaves and comes back every time you sneeze, or whenever you yawn, or even when you are startled. The Nahua believed that it was not good to sneeze and keep talking, because it causes your tonalli to leave and once your Tonalli leaves, you have to wait for a period of time before it returns. At that moment, anything can enter your body. However, the soul of the heart (Teyolía) and the soul of the liver (Ihiíyotl) only leave your body when you die; those two souls will exit only at the exact moment of your death.