Questions & Answers about Voy a quedarme dormido en el sofá.
Voy a + infinitive is a very common way to talk about the near future in Spanish, similar to I’m going to ... in English.
So voy a quedarme dormido means the speaker expects this to happen soon, or feels it is likely to happen.
You could also use the simple future, such as me quedaré dormido, but in everyday speech voy a ... often sounds more natural and conversational.
Quedarse dormido is a very common expression meaning to fall asleep.
Even though quedarse by itself often means to stay or to remain, in this expression the whole phrase has its own meaning. It suggests that someone ends up asleep, often without really planning to.
So it is best to learn quedarse dormido as a set expression.