If I just lift the sheet over my head for more than 60 seconds I’m gasping for air, but my cat and both Chihuahuas can sleep peacefully under a sheet and heavy quilt for hours without surfacing!
Does ther heart rate go into hibernation mode?
Do they just pass out for a while?
It baffles the mind.
Y’know, I wonder the same thing. My pug loves to dive under the blankets, and will sleep under them for hours with no problems. When she was a puppy, I worried, but she’s been doing it for 10 years with no issues.
You’re imagining that feeling of suffocation. A sheet is not going to impede you from getting necessary airflow and oxygen.
While I don’t generally enjoy breathing warm air under blankets, when it’s cold as hell outside of them (camping, broken furnace) I’ve slept safely and comfortably with my head under several layers.
maybe it’s genetic but I frequently find the chil’ens or myself totally under our respective blankets. it’s warm in the winter and darker in the summer when we sleep outside.
Smaller creatures need a lot less air, I imagine.
What gets me is how our cats can sleep under the covers and not gag. I tend to fart in my sleep (and when awake, for that matter) and unless I blast one right in their faces, the girls never budge. Weird.
That isn’t normal. Unless your sheets are made of plastic, there shouldn’t be any reason you can’t breathe under them.
Smell cat food then ask yourself that question again.
Point taken. 
I’m not necessarily saying that this is what’s going on here, but there are major individual differences in sensitivity to CO2. Anxiety-prone people can be hypersensitive to CO2 challenge.
I used to sleep under the blankets when I was a child. But I can’t do it anymore, and I know it’s purely pyschological.
For me it’s claustrophobia. I can’t stand anything over my face like that. Doesn’t bother my cat at all, he’s under the comforter right now.
Not a doctor or any other relevant occupation, but the covers might cause the CO2 level to go up a bit since your exhaled breath takes longer to escape. It then becomes an issue of how sensitive you and your critters’ breathing reflexes are to the elevated CO2.
I never thought to wonder about this. I sleep with my head under the covers in the winter, sometimes.
I have one cat that demands to sleep under the blankets with me, but she keeps her head out, because she thinks she’s people.
I’ve done the sleeping with my head under the covers while camping thing. Just don’t do it if you have on OFF! (mosquito repellent)- you’ll get sick as a dog.
This, IMO, pretty much sums it up.
The feeling of suffocation comes from a buildup of CO2 rather than a lack of oxygen.
From the Wiki
So while you’re still getting plenty of oxygen, the extra CO2 is triggering the suffocation feeling. I presume you could de-sensitize yourself if you were so inclined by simply spending more time with your head under the covers.
Or just make yourself completely psychotic from the mental stress. Roll the dice. 
I sleep with my head and nose under the covers frequently, there is plenty of air. It’s sort of a humidifier too, keeps my nasal passages and throat from drying out in the winter as much.
In cold weather, my dog lives for the moment around 3 am when I will flip the covers back and let him lay down beside me, then flip the covers back completely over him. His head is on my chest and the last sound he makes for hours is a contented whuuumpf. Now, I can see a cat or small dog surviving on just the trapped air for hours… but you’d think a Dane would run out of oxy pretty quickly. Guess not.
My husband likes to sleep with a blanket over his face and our daughter somehow inherited this tendency. It drives me CRAZY, worrying about her smothering under there. Good to hear that many critters do it regularly and survive.
I sleep with the sheet, a wool Hudson’s Bay blanket and a comforter over my head. I breathe fine. My cats, on the other hand, don’t like to be under the blankets.