Question

why is it harmful to cover our face while sleeping

Answer 1

It is harmful to cover our face while sleeping because , when we cover our face we are not able to take fresh Oxygen , we take in CO₂ and release CO₂  . Due to which the CO₂ level in our body or blood increases and we feel suffocation. 
It may lead to dead  or many other problems .
So , we should never sleep by covering our face.

Answer 2

we take our co2 when we close our face.so it is harmful to close our face while we sleep.