Figure 1.

A, Effects of retrieval expectancy on retention of declarative memory for word pairs across 9 h intervals filled with nocturnal sleep (Sleep), nighttime wakefulness (Night-Wake), and daytime wakefulness (Day-Wake). Retention performance is indicated by the percentage of word pairs recalled at retrieval, with performance on the criterion trial during learning set to 100%. Mean (±SEM) retention performance is indicated for subjects who expected (expected group, black bars) or did not expect retrieval testing (unexpected group, open bars), as well as for subjects who suspected the retrieval although they were never explicitly informed about it, i.e., these subjects were originally assigned to the unexpected group (gray bars); *p < 0.05, ***p < 0.001, for differences between groups. B, Subjects in each group learned (L) a paired-associate task in the evening (22:00–23:00 h) or the morning (08:00–09:00 h), and half of the groups (expected) were then informed that recall (R) would be tested after a 9 h retention interval (i.e., at 08:00 or 18:00 h), whereas the other groups were not informed about the retrieval test (unexpected).