  Humans have been aware for thousands of years that sleep comes in many forms,
accompanied by different kinds of mental content. We review the first-person
report literature on the frequency and type of content experienced in various
stages of sleep, showing that different sleep stages are dissociable at the
subjective level. We then relate these subjective differences to the growing
literature differentiating the various sleep stages at the neurophysiological
level, including evidence from electrophysiology, neurochemistry, and
functional neuroimaging. We suggest that there is emerging evidence for
relationships between sleep stage, neurophysiological activity, and subjective
experiences. Specifically, we emphasize that functional neuroimaging work
suggests a parallel between activation and deactivation of default network and
visual network brain areas and the varying frequency and intensity of imagery
and dream mentation across sleep stages; additionally, frontoparietal control
network activity across sleep stages may parallel levels of cognitive control
and meta-awareness. Together these findings suggest intriguing brain-mind
isomorphisms and may serve as a first step toward a comprehensive understanding
of the relationship between neurophysiology and psychology in sleep and
dreaming.
