During Anapana, why does the breathing become slower and slower as we progress? I've been practicing mindfulness of breath (Anapanasati) lately and noticed this phenomena. During the initial few minutes, the breathing is generally short, but as the mind starts concentrating and gets still, the breathing becomes slower and slower. In fact, some times, I feel like one in/outer breath is taking like minutes. What actually causes this phenomena?
At least two things cause this phenomena: (1) the mind not desiring &amp; not thinking, which results in the restless energy of desire &amp; thinking not making the breathing restless &amp; agitated; and (2) what Buddhism calls the 'Nirvana element' ('nirodha dhatu'), i.e., what science calls 'homeostasis', which, like the force of 'gravity', moves the breath towards the equilibrium or peace/stillness of Nirvana. 

Whenever the mind is free from craving, clinging &amp; desiring, the mind &amp; the breath will enter into the 'stream' to Nirvana. It is the same as when a drug addict enters into 'cold turkey'. Natural forces purge the body of the drug addict of its toxins. Similarly, the 'cold turkey' of meditation dissolves the agitations of conditioned mental formations stored in the mind, body &amp; breath, making the breath more calm &amp; pure.