When I was a new diver, I would go through air pretty quickly--I never had the lowest air among the divers back on the boat after a dive but was usually pretty close. Over time, once I got the hang of maintaining neutral buoyancy, staying streamlined, and efficient finning, my air consumption improved dramatically. I would still use significantly more air than my wife, who is also my dive buddy, but she is smaller than me and weight and gender are factors in air consumption. Nevertheless, I wondered if I could further improve my air consumption so she'd be able to get a full tank's worth of diving (instead of having "plenty" of air left in her tank when the volume in my tank required us to head up).
So the last time we went on a dive trip, I started consciously focusing on the rate at which I was breathing and trying to maintain a steady rate of respiration with full breaths instead of more rapid shallow breathing. When I did that, my air consumption plummeted. At the end of the first such dive, I had significantly more air than my wife. Over the next few dives, my air consumption was quite low when I focused on my breathing but creeped back up to my prior consumption level when some other aspect of the dive commanded all of my attention.
I also started having occasional but intense post-dive headaches on this trip. After a while I realized the headaches only occurred after dives on which I'd been particularly "good" with my air consumption.
So here's my question- is it possible to consciously (albeit unintentionally) suppress your breathing rate to the point of causing O2 deprivation or CO2 buildup that could lead to such headaches? I was trying to maintain a steady/healthy rate of deep breathing but would think that even if I accidentally breathed too little for a few minutes, my body would react by gasping for air or panting for breath or something (which never happened on these dives). When I'm engaged in heavy physical exertion on land, I breathe more whether I want to or not because my body needs the air. Wouldn't the same thing happen while diving?