Daydreaming brain and sleeping brain states are quite different from one other.
We have a brain structure, more accurately, a type of neural network formed in parts of the brain, called the “Default Mode Network”.
This network, linking several parts of cortical areas and the limbic system, which are known to be involved in sensory experiences. When this network is active, as we learned from Buckner et al, the individual is not focused on outside stimulus, but instead is turned inside, hence the daydreaming. (More accurately called Mind-wandering ) When this default network is active, it provides its own stimulation. In layman’s terms, it’s entertaining us, but we are not far away from our wakeful state.
Sleeping, on the other hand is a complex state of entire organism that plays a key biological role such as building up or the repair of immune and muscular systems as well as other syntheses. To be absolutely fair, we are not crystal clear on how the sleeping mechanics of the brain interacts with each other. However, we know that mostly by virtue of the VLPO and thalamus of our brain, a cornucopia of neurotransmitters are controlled, which is assumed to help our brain switch between sleeping and wakeful states. When sleep occurs, a variety of signals of wakefulness are interrupted and most outside stimuli is blocked, which is quite different from what happens in the state explained before.
Just to add on to this: it’s known that what is observed as daydreaming in autistic people may be an altogether different process neurologically, so there is probably some variation in people based on what their neurologies are, even if there isn’t much of a difference in what they describe themselves as doing.
There’s a lot we do understand there.
There have been many studies of how the Default Mode Network functions in sleep. In general, as we descend into sleep, activity of this network decreases, especially in prefrontal cortex (which may account for loss of logical thinking during sleep).
Interestingly, there are key differences in the functioning of the Default Mode Network and brain activity in general between REM and NREM sleep. While dreaming occurs in both NREM sleep (people report dreaming ~30% of the time when awoken from NREM) and REM sleep (~90% of the time), the most vivid and interesting dreams occur in REM sleep. During REM sleep, the Default Mode Network is largely reconstructed, and large-scale brain activity, as measured by EEG, is actually quite similar to wakefulness (high frequency, low amplitude waves) and very unlike NREM sleep (low frequency, high amplitude waves).
People have speculated on a possible link between daydreaming and dreaming since Freud, but the underlying mechanisms are still not entirely clear for either phenomenon. Certainly there are some similarities in functional brain activity, however.