Recovery sleep after sleep deprivation almost completely abolishes dream recall

The study investigated the effect of one night of sleep deprivation on dream recall at morning awakening after recovery sleep. Forty healthy subjects were studied after adaptation (A) and baseline nights (B), and a recovery (R) night following 40 h of prolonged wakefulness. Parallel to the well-known recovery sleep changes (slow-wave sleep--SWS--rebound, decreased number of awakenings and of REM sleep amount), an almost complete abolition of dream recall was found, with an around 75% decrease with respect to the adaptation and baseline nights. The number of dreams recalled by those subjects with successful recall (REC) did not significantly differ between nights. Moreover, gender and sleep stage at awakening did not affect either the proportion of REC subjects or the number of dreams recalled by REC subjects during each night. Finally, the drastic impairment of dream recall after R night was associated to a larger increase of SWS and a shorter REM sleep duration. We suggest that dream recall could have been impaired during R night because: (i) the lower number of spontaneous awakenings over the night reduced the contents available in memory as possible cues for the retrieval of dream experiences at morning; (ii) mental experiences, having been elaborated during SWS more than in the other nights, were less dreamlike (i.e., perceptually vivid and bizarre) and, thus less accessible at morning recall than those elaborated during the nights with a higher proportion of REM sleep; (iii) dream contents, as a peculiar type of episodic information, were less consolidated because of the lower effectiveness of declarative memory during recovery sleep.