When the sleepy participants’ eyes closed in “microsleep,” a.k.a. dozing, the researchers saw reduced activity in the thalamus, the part of the brain responsible for relaying sensory and motor signals to other parts of the brain. The thalamus is also responsible for regulating sleep (and the same effect was found in the scans of rested participants) so this part isn’t very surprising.
But more surprisingly, the researchers saw increased activity in the parts of the brain associated with sensory processing, which could account for the vivid images that seem to spring up when you first drop off to sleep. The researchers also observed more activity in the frontal parietal lobe, responsible for helping you pay attention, likely as a result of the brain’s attempt (and failure) to obey the “stay awake” command.