Dreaming in the late morning: summation of REM and diurnal cortical activation.

Dreaming is more intense during REM sleep than other sleep stages, but it remains unclear whether the brief, phasic events of REM or its sustained, tonic background activation are responsible for features like visual vividness, narrative shifts, and incongruities. This experiment sampled reports of imagery and thought during spontaneous variations within the REM-NREM cycle and across the 24-hour diurnal rhythm, manipulating sleep and wake times to separate their effects. Results suggest that the main driver of vivid visual imagery and enhanced cognitive activity during sleep is a pattern of subcortical and cortical activation common to both the REM phase and the activated (waking) phase of the diurnal cycle.

Study at a glance

Abstract

Since the discovery that the characteristics of dreaming sleep are far stronger in Stage 1 rapid eye movement (REM) sleep than in any other biological state, investigators have attempted to determine the relative responsibility of the tonic versus the phasic properties of REM sleep for the different characteristics of dreaming--features such as the amount of information in the dream report, the brightness and clarity of the visual images, shifts in thematic continuity, and incongruities of image and meaning. The present experiment is designed to identify dream characteristics that are specifically associated with tonic changes in level of cortical activation within sleep. It samples reports of imagery and thought during spontaneous variations within one phase of the 24-h diurnal rhythm and across the REM-NREM (non-REM) sleep cycle in order to identify the independent and joint contributions of the two cycles to imagery and thought. The rising phase of the diurnal cycle in the late night and morning was estimated from clock time during the late night and early morning and was varied by delaying the sleep onset and waking time of the subjects. Considered together with other studies, the results suggest that the major determinant of vivid visual imagery and enhanced cognitive activity during sleep is a pattern of subcortical and cortical activation that is common to both the REM phase of the REM-NREM cycle and the activated phase of the 24-h diurnal wake-sleep cycle.