Patients with complex partial seizures recall dreams less often after stage 2 NREM sleep than after REM sleep, regardless of which hemisphere the seizure focus is in. The length and structural organization of dreams do not differ significantly between REM and NREM sleep. However, during REM sleep, the length of story-like dreams is influenced by global cognitive functioning. These results suggest that the elaboration of dream experience is maintained in both sleep stages, but access to information for conversion into dream content and the consolidation of that content is less effective during NREM sleep. Further research may determine whether impaired memory functioning during NREM sleep is a side effect of anticonvulsant treatment.
Dream reports occur after awakenings from all sleep stages, not just REM sleep. In a nap study with 10 healthy young adults, dream recall length, measured by total word count, was greater after REM sleep than after brief NREM sleep periods (10 seconds and 1 minute), but recall length did not differ among NREM awakenings of varying durations. Delta EEG activity, an indicator of sleep depth, was lower after REM than after longer NREM periods, while Beta activity did not vary. The consistent recall length across NREM conditions suggests that arousal level alone does not determine dream report length; memory processing or limitations of EEG as an arousal measure may explain the longer REM reports.
Twenty-two young adults were awakened during REM sleep to test whether the length of prior REM sleep affects dream recall. Participants were awakened after either 5 or 10 minutes of REM sleep during the second and fourth REM periods of the night. The number of words used to describe dream mentation did not differ between 5- and 10-minute awakenings. However, reports from the fourth REM period contained nearly twice as many words as those from the second REM period. The findings suggest that recall from REM awakenings depends on central arousal level and its influence on memory processes.