Sleeping with a mask may result in a lack of oxygen, but usually not to the extent of suffocation. When you sleep at night, the indoor air itself will be worse than the outdoor air. Wearing a mask may restrict breathing to some extent and affect oxygen intake, so you may wake up in poor spirits. However, as the mask itself has good breathability, it is usually not so severe as to cause suffocation. However, it is not recommended to wear a mask at night if sleeping is not necessary.