Memory sources of REM and NREM dreams

Sixteen male volunteers slept 4 nonconsecutive nights each in a sleep laboratory. They were awakened for one dream report per night. Awakenings were made, in counterbalanced order, from early-night and late-night rapid-eye movement (REM) and non-REM (NREM) sleep. Following dream reporting, subjects were asked to identify memory sources of their dream imagery. Two independent judges reliably rated mentation reports for temporal units and categorized memory sources as autobiographical episodes, abstract self-references, or semantic knowledge. We replicated earlier findings that semantic knowledge is more frequently mentioned as a dream source for REM than for NREM reports. However, with controls for length of reports, the REM-NREM difference disappeared, indicating that the stage difference in memory sources was not independent of stage difference in report lengths. There was a significant effect of time of night on source class, but only in REM sleep: Both without and with controls for report length, more semantic sources were cited for late than for early REM dreams.