Independent from physical activity, your state of mind influences your breathing patterns. In addition, your breathing also reveals patterns in sympathetic and parasympathetic arousal associated with different emotional and cognitive states (e.g. calm, focus, etc.).
Spire's design and algorithms are based on the research of our scientific advisor, Dr. Stephen Porges. Dr. Porges is the originator of the polyvagal theory, which describes the three evolutionary layers (evolved mammalian, mammalian, and reptilian/amphibian) of our nervous system used to maintain homeostasis via the longest cranial nerve, the vagus.
The vagus nerve is responsible for regulating heart rate, speech, digestion, as well as cortisol and adrenaline levels. Dr. Porges spearheaded the use of respiratory sinus arrhythmia (RSA) as a measure of vagal tone. Conscious control of breathing modulates the extent to which the vagus is gated.
Your respiratory rate tells Spire if you are in a period of physical or mental exertion. A faster rate of breathing indicates your body needs more oxygen, and thus is working harder. This can be used to measure cognitive and physical activity.
Variability in breathing also ties in closely with focus. A consistent degree of cognitive demand reflects a level of concentration. In this state, the variability of your breathing rate is decreased. Your breathing exhibits small hiccups, speed-ups, pauses, and slow-downs when you are worried or tense; Spire senses these.