Sleep's role in the processing of unwanted memories.

The concept of 'repression' dates back to Freud, assuming that undesirable memories can become suppressed and that dreams ease repression by permitting these memories to be reinstated. Here, we followed this idea adopting the 'directed forgetting' approach of experimental psychology. The voluntary suppression of unwanted memories results in impaired later retrieval. Because sleep is known to benefit consolidation of newly learned materials, including cognitive skills, we hypothesized that memory suppression would be enhanced by sleep, and perhaps particularly by rapid eye movement (REM) sleep, which is associated more often with dream reports. Subjects (n=42) learned a list of word-pairs and, subsequently, the first (cue) words of the pairs were presented again; for half these words subjects had to recall respective second words (response pairs) and for the other half they had to keep respective second words out of mind (suppression pairs). Retrieval of both response and suppression pairs was tested after 8h of sleep or wakefulness (main experiment) or after 3-h periods of early slow wave sleep (SWS)-rich or late REM-rich sleep (supplementary experiment). Response pairs were generally recalled better after sleep than wakefulness (P<0.05). Recall of suppression pairs was, as expected, worse than of response pairs. Contrary to our hypothesis, memory for suppression pairs was not affected differentially by sleep. In the supplementary experiment, compared to SWS-rich sleep, REM-rich sleep even improved recall of suppression pairs (P<0.05). Thus, sleep does not benefit the forgetting of unwanted memories but, on the contrary, REM sleep might even counteract the voluntary suppression of memories making them more accessible for retrieval.