No, not true. I exclusively breathe through my nose all day (unless panting from a workout). The moment I lay down, something changes and my nose stuffs up. This has only gotten worse over time. If I were to, say, tape my mouth shut, I would absolutely suffocate in short order.
You might want to really clean your bed (pillow, sheets etc) and surroundings from potential allergens like dust mites, mold, pollen etc. Ideally with one of these mattress vacuum cleaners. Window sills (even outside, if the window doesn’t seal hermetically) might leak mold spores or other particles into your bedroom. A really good air filter next to the bed might also help.
>>"something changes"
When you lay down, blood shifts to your head which can cause your nasal passages to become engorged with blood, causing a stuffy sensation.
