Daytime mind wandering is linked to dream content while asleep, study finds

Our default waking mode (and also during dreaming phases of sleep) is not a brain at rest, but what is known as mind wandering.

I think it is more helpful to talk about the underlying processes: spontaneous thought, or the decoupling of attention from perception, which is what happens when our thoughts separate from our perception of the environment. Both these processes take place during mind-wandering and daydreaming.