Sleep may partly exert a positive influence on memory through the processing underlying the transformation of items of declarative knowledge into contents of mental sleep experience (MSE). This hypothesis implies that the level of consolidation (and thus, long-term retention) should be enhanced for those items which are repeatedly processed and transformed into identical or very similar (so-called interrelated) contents of distinct MSEs developed over the same night. To test this prediction, we examined accessibility at delayed recall (i.e., the next morning) of the interrelated contents of MSEs reported (immediate recall) by 14 subjects who were awakened during the first four periods of rapid eye-movement (REM) sleep in two experimental nights. Interrelated contents were much more frequent, and at delayed recall much better retained, than other, non-interrelated contents in report pairs of MSEs. Moreover, they were also more frequent and better retained than by-chance similar or identical contents, as estimated in report pairs of MSEs of different subjects. These findings provide partial but coherent evidence in favour of the hypothesis that a generation effect occurs during sleep, with a further consolidation of the input and the output of MSE processing (respectively, the items of declarative knowledge and the contents of MSEs resulting from their elaboration during sleep).