During the transition to sleep, reflective awareness—the ability to recognize experiences as internally generated—breaks down before phenomenal awareness, the basic experience itself, fades. Dream-like experiences, characterized by uncontrolled thinking and perceptual images mistaken as real, are associated with an inverse relationship between cognitive effects and physiological activation in the brain. In 45 healthy young subjects, EEG microstate analysis showed that dream-like experiences correlated with increased presence of a microstate sourced in the superior and middle frontal gyrus and precuneus, and decreased presence of a microstate linked to higher-order visual areas. This suggests disengagement of cognitive control systems mediated by specific inhibitory microstates.
The ability to organize self-generated thought into coherent, meaningful semantic representations is a central aspect of human cognition and undergoes regular alterations throughout the day. To investigate whether changes in semantic processing might explain the loss of coherence, logic, and voluntary control over thinking typically accompanying the transition to sleep, N400 evoked potentials were recorded from 44 healthy subjects. Auditory word pairs with varying semantic distance were presented while subjects were allowed to fall asleep. Semantic distance reliably elicited an N400, and lower wakefulness levels were associated with increased frontal negativity. Contrary to the initial hypothesis, the results showed an increased N400 effect with decreasing wakefulness, suggesting that semantic processes alone may not explain diminished thought control during sleep onset.