It's a myth that you die in real life if you die in your sleep

I believe if you die in your dream. You are actually dead.

Because no one knows how death is, and dreams come from personal experiences and such events that are manipulated by your brain. (If you had a dream with spongebob it is because you have seen him).

So to dream that YOU die is impossible.
If you dream someone else dying that is normal because you have seen many people die, in life, movies, etc.