Sleep on it

Another of the papers in PNAS online this week is this one:

Human relational memory requires time and sleep
Jeffrey M. Ellenbogen et al.
Relational memory, the flexible ability to generalize across existing stores of information, is a fundamental property of human cognition. Little is known, however, about how and when this inferential knowledge emerges. Here, we test the hypothesis that human relational memory develops during offline time periods. Fifty-six participants initially learned five "premise pairs" (A>B, B>C, C>D, D>E, and E>F). Unknown to subjects, the pairs contained an embedded hierarchy (A>B>C>D>E>F). Following an offline delay of either 20 min, 12 hr (wake or sleep), or 24 hr, knowledge of the hierarchy was tested by examining inferential judgments for novel "inference pairs" (B>D, C>E, and B>E). Despite all groups achieving near-identical premise pair retention after the offline delay (all groups, >85%; the building blocks of the hierarchy), a striking dissociation was evident in the ability to make relational inference judgments: the 20-min group showed no evidence of inferential ability (52%), whereas the 12- and 24-hr groups displayed highly significant relational memory developments (inference ability of both groups, >75%; P < 0.001). Moreover, if the 12-hr period contained sleep, an additional boost to relational memory was seen for the most distant inferential judgment (the B>E pair; sleep = 93%, wake = 69%, P = 0.03). Interestingly, despite this increase in performance, the sleep benefit was not associated with an increase in subjective confidence for these judgments. Together, these findings demonstrate that human relational memory develops during offline time delays. Furthermore, sleep appears to preferentially facilitate this process by enhancing hierarchical memory binding, thereby allowing superior performance for the more distant inferential judgments, a benefit that may operate below the level of conscious awareness.

This suggests two things to me:

1. It actually should work to think about a problem in the evening, go to bed, and then think about it again in the morning. This actually seems to work fairly well for me, so I find it entirely plausible.

2. The standard 15-minute scientific talk is just the right length for nobody to remember anything at the end.

This passage from the discussion is a mini-review:

It is interesting to note the similarity between this finding and recent evidence implicating sleep in the enhancement of memory associations (23), the development of flexible, creative information processing (24, 25), and the relational building of component motor-sequence memories (15, 17, 18). Together, these data provide a new and emerging role for sleep in facilitating associative integration of information, a form of memory binding or extracting experience generalities. A potential candidate structure orchestrating these associative effects might be the hippocampus. Numerous studies have emphasized the dependence of transitive inference on the hippocampal integrity (1). Considering that the hippocampus has consistently been implicated in offline memory reprocessing, manifest in neuronal "replay" following learning (e.g., see ref. 26), a speculative hypothesis is that similar neural reactivation during offline periods of wake and (especially) sleep facilitates relational mapping between learned items. Therefore, such offline hippocampal reprocessing may underlie not only the strengthening of individual item memory, but the binding, and hence subsequent flexible use and expression, of acquired declarative memories (Ellenbogen et al. 2007:7727).

They also cite some research that indicates that people can learn relational hierarchies without being consciously aware of them, which seems relevant to social cognition in non-human primates as well as humans.


Ellenbogen JM, Hu PT, Payne JD, Titone D, Walker MP. 2007. Human relational memory requires time and sleep. Proc Nat Acad Sci USA 104:7723-7728. doi:10.1073/pnas.0700094104