A lot of people simply pass away in their sleep.Maddie? she was the one that voiced meowth? how did she die in her sleep?
It could be a Heart Attack, or it could just be that the mind wasn't working hard enough to keep the person alive. Since when your asleep, your technically knocked out, your mind must work overtime to keep the body going. But if it doesn't work as hard as it should, the person would simply pass away.
My Grandfather, Uncle and Father all went the same way...
RIP Maddie Blaustein.