If you go to sleep with a completely blocked nose, you can die from asphyxiation in your sleep. This infamously happened to Mark Gleeson. If your nose is completely blocked from a cold be sure to stay awake for as long as you have a blocked nose. The reflex to breathe through your mouth when you have a blocked nose does not happen during sleep.
I looked on YouTube for videos of people pinching people's noses shut while they sleep and I can confirm none of them opened their mouths and breathed, instead they just woke up, imagine if they were in deep sleep though and they had a bad cold.
Myths like this are dangerous and can get people killed. There's no scientific evidence that a person will open their mouth and breathe through their mouth if their nose is blocked during sleep. In fact, babies born with agenesis of the nose suffocate shortly after being born.
I looked on YouTube for videos of people pinching people's noses shut while they sleep and I can confirm none of them opened their mouths and breathed, instead they just woke up, imagine if they were in deep sleep though and they had a bad cold.
Myths like this are dangerous and can get people killed. There's no scientific evidence that a person will open their mouth and breathe through their mouth if their nose is blocked during sleep. In fact, babies born with agenesis of the nose suffocate shortly after being born.