Edit: Perhaps immersive psychedelics (LSD, ketamine) are similar actually. Speculatively, one could define "real" as our ability to doubt, "fantasy" as the suspension of doubt.
In this framework dreaming is when the evidence collecting side becomes a lot less rigorous than it usually is, so it says "sure, why not" without reference to a stimulus (i.e. a way to be falsified). In other words it's the brain's predictions continuously coming true.
SlateStarCodex has a nice article[1] about this.
> The key insight: the brain is a multi-layer prediction machine. All neural processing consists of two streams: a bottom-up stream of sense data, and a top-down stream of predictions. These streams interface at each level of processing, comparing themselves to each other and adjusting themselves as necessary.
> ...
> You’re not seeing the world as it is, exactly. You’re seeing your predictions about the world, cashed out as expected sensations, then shaped/constrained by the actual sense data.
[1] https://slatestarcodex.com/2017/09/05/book-review-surfing-un...