system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: we all know that when someone tells us to breathe naturally, we're suddenly aware of our breathing action. What is the physiological and psychological difference between our normal breathing cycle and the active deep breathing that we do deliberately?
gpt
We definitely have a great understanding of the general mechanisms of breathing, but it's pretty advanced for ELI5. Essentially, there is a small part of the brain, deep and in the back, near the top of the neck, that controls unconscious breathing patterns. To condense decades of research into a single sentence, it's essentially just a self activating, pulsing brain signal; on = breathe in, off = breathe out (or vice versa, it's been a while).

The conscious mind can send signals further down the neural pipeline to override that gentle rhythmic pulsing, allowing you to hold your breath, breathe slower or faster, and deeper or shallower. 

The pulsing region itself can be modified by the environment and semiconscious efforts; sleep and relaxation drastically reduce breathing rate, while stress and excitement increase it, while still having your breathing under unconscious control.

In terms of actual breathing, AFAIK there is no difference in things like air volume or flow rate that can be solely attributed to conscious vs unconscious breathing.