Dream recall upon awakening from NREM sleep is predicted by reduced delta activity in left frontal and temporo-parietal areas during the final segment of sleep. This finding suggests that increased cortical activation is a prerequisite for remembering dreams from NREM sleep, challenging the traditional view that dreaming is unique to REM sleep. The study used a multiple nap protocol and within-subjects design to collect dream reports after stage 2 NREM sleep awakenings. The results indicate that the brain mechanisms underlying dream recall during NREM sleep may share features with those involved in encoding episodic memories during wakefulness, though they also depend on the specific electrophysiological state of sleep.
Dream recall after REM sleep is linked to higher frontal theta oscillations, a brain rhythm also involved in encoding memories while awake. In six healthy subjects awakened from afternoon naps, theta activity in the frontal cortex was greater before a remembered dream than before a forgotten one, though only the measure of sustained theta oscillations reached statistical significance. The findings support the idea that dream recall depends on the momentary brain state during sleep rather than on a fixed individual trait.
Recent findings confirm that the neurophysiological mechanisms for encoding and recalling episodic memories are largely comparable across wakefulness and sleep. Brain lesion and neuroimaging studies indicate that the temporo-parieto-occipital junction and ventromesial prefrontal cortex play a crucial role in dream recall. Morphoanatomical measurements reveal direct relations between volumetric and ultrastructural measures of the hippocampus-amygdala and specific qualitative features of dreaming. Intracranial recordings in epileptic patients support the notion that hippocampal nuclei mediate memory formation during sleep as in wakefulness. Surface EEG studies show that sleep cortical oscillations associated with successful dream recall are the same as those involved in encoding and recall of episodic memories during wakefulness. These converging pieces of evidence strengthen the view that episodic/declarative memory mechanisms may be the same across different states of consciousness.
Dream recall after awakening is linked to specific brain oscillations that differ between REM and NREM sleep. After REM sleep, higher frontal theta activity (5-7 Hz) predicts successful dream recall, mirroring the brain activity seen when encoding episodic memories while awake. After stage 2 NREM sleep, lower alpha activity (8-12 Hz) in the right temporal area predicts successful recall. These findings suggest that the neurophysiological mechanisms for encoding and recalling episodic memories may be consistent across different states of consciousness.
After 40 hours of prolonged wakefulness, a recovery night of sleep almost completely abolished dream recall, reducing it by about 75% compared to adaptation and baseline nights. The number of dreams recalled by those who did remember dreams did not differ significantly between nights, and gender or sleep stage at awakening did not affect recall. The drastic impairment of dream recall was associated with a larger increase in slow-wave sleep and shorter REM sleep duration. The authors suggest dream recall may be impaired due to fewer spontaneous awakenings, less dreamlike mental experiences during SWS, and poorer consolidation of dream content as episodic memory during recovery sleep.