Morning recall of verbal material depends on prior sleep organization.

Despite the evidence that sleep may facilitate memory, controversial findings concern the role of sleep states (NREM, REM). We put forward the hypothesis that sleep organization, i.e. the regular occurrence of NREM-REM cycles, more than sleep states per se, may be crucial for the retention of verbal material presented before sleep. An experiment was performed in which recall of verbal material was requested of young subjects after three different kinds of night sleep: undisturbed sleep, fragmented sleep without sleep cycles disorganization, and fragmented sleep interrupted with sleep disorganization. Morning recall of verbal material was impaired after the night with disturbed sleep cycles, whereas it was not after the night with preserved sleep cycles; the amount of REM was similar in both cases. We conclude that the recall of verbal material is greatly affected by sleep cycle disorganization.