What would happen if you threw up while sleeping face up?

Inhale vomit. If someone vomited during the night they would normally awaken first. It is quite rare for a non-intoxicated person to spontaneously vomit, mid-sleep. However, this is common in the setting of intoxication with any sedating drug (e.g. Alcohol, benzodiazepines, barbiturates), whereby the person is so sedated that they don't awaken when vomiting, resulting in aspiration of vomit, which can be fatal.