Autoregulation of carbon dioxide is extremely precise in the mammalian organism. What should surprise you is how little carbon dioxide production decreases under anesthesia. It would be even less during sleep. Metabolism is primarily affected by body temperature and exercise, with the latter dominating.
Hence, the mandate to relax while diving. Given the various inefficiencies in our equipment, it becomes easier to see why exertion on a rebreather can be fatal.
Like many biologic systems, RMV data follows a bell curve of one shape or another. I wish my SAC were lower, but it just ain't. So I work really hard on streamlining, finning technique and resting. I am
really tempted to try skip breathing since I know I won't get hypoxic, but as a physician I know that way leads to catastrophe. And as a diver wanting to start CCR, I absolutely have to resist the temptation.
My comments above were prompted by seeing on a daily basis the adjustment in end-tidal carbon dioxide that occurs when I change ventilation in my patients. In the early days of end-tidal CO2 monitoring, truly frightening instances of hypercarbia were seen with what were assumed to be "adequate" ventilation, along with more than one cardiac arrest.
So do all those things you were taught, to relax and streamline. Breathe fully (not shallowly) to minimized dead-space ventilation, and accept what you get. We just can't all be like
@scubadada and Crusader, sad to say.