Memory and Brain Health: Sleep and Memory

[morgan_ratner]

Written and contributed by Mark Gluck, author of Learning and Memory.

It has long been assumed that sleep, beyond its role in rest and refreshment, also has a facilitating effect on learning and memory. After all, who doesn’t recognize the experience of working fruitlessly on a problem to the point of exhaustion, and then coming up with a sudden solution following a night of sleep? It is only over the last two decades that the effect of sleep on cognition has been studied in depth by the scientific community. This research has led to a growing understanding of how sleep affects learning and memory.

A typical night of sleep includes cycles of alternating sleep stages, each characterized by a different profile of brain activity. In one of these stages, rapid-eye-movement (REM), or paradoxical, sleep,  the brain seems as active as when awake. This is the stage in which most dreams appear. Another stage, slow-wave sleep (SWS), or deep sleep, is characterized by highly synchronized brain-neuron activity and is the stage in which it is most difficult to wake the sleeper up.

Recent research has combined sophisticated experimental designs, neuroimaging, and single-cell recordings to discover the relative impacts of the different sleep stages on various cognitive functions. It shows that SWS is mostly important for the consolidation of declarative memory, as well as for rule learning, spatial navigation, and insight. REM sleep, on the other hand, exerts its greatest effect on procedural memory. Some of the mechanisms by which sleep contributes to these functions are also becoming clear. During SWS, the hippocampus, a brain region involved in episodic and associative learning, ”replays” some of the waking experiences acquired during the previous day. The replay allows the new memories to be reorganized into a more efficient structure and also contributes to their assimilation into the general knowledge store of the individual. This reorganization is reflected as better performance the following day.

Many questions are still unanswered. For example, the mechanism by which REM sleep facilitates learning and memory remains somewhat less characterized in comparison to that of SWS. However, the rapid advancements in this field hold out promise that the years to come will bring a fuller account of why the third of our lives we spend sleeping, a seemingly wasteful behavior from a learning–memory perspective,  is in fact not wasted at all.

FURTHER READING

Stickgold, R., & Walker, M. P. (2005). Memory consolidation and reconsolidation: What is the role of sleep? Trends in Neuroscience, 28, 408–415.

Diekelmann, S., & Born, J. (2010). The memory function of sleep. Nature Review Neuroscience, 11, 114–126.