Many of us wish we had a better memory, particularly the night before a big exam or project. Conventional wisdom tells us to “get a good night’s sleep,” but this old adage seems outdated in our fast-paced world. The purpose of this summary is to review the neurobiological evidence supporting “a good night’s sleep” as one of the critical supports to optimize learning and memory. And, the good news is that many aspects of our sleep routine and schedule are under our control and can be improved with disciplined practice, thereby improving our sleep quality and memory.
We spend approximately 1/3 of our lives sleeping, but that time remains inaccessible to our consciousness making the functions of sleep somewhat mysterious. Nevertheless, recent neuroscience research has made impressive progress in clarifying the nature of sleep and its functions, highlighting its role in learning and memory. New memories are fragile until they are “consolidated,” meaning that the memory is “stabilized and stored” (Hernandez, 2011) and integrated with existing memories, facilitating retention and later recall. Sleep is critical to this consolidation process. If you think about the vast array of experiences and information that we encounter in our day, and how much of it ‘falls out’ of memory soon after, it is not surprising that a consolidation process is needed to store and retrieve this information efficiently. A technology metaphor may serve to illustrate the purpose of consolidation. Think of a specific piece of information that you encountered in your day and would like to remember as akin to a useful website that you have discovered while surfing the internet. This website is important and you would like to remember it for the future, but the challenge is that this website is embedded within a complex array of additional information (i.e., the internet). Consolidation is akin to the process of moving this website to a bookmark folder, organizing it with other related websites, and tagging it for easy retrieval. Without this consolidation process, it would be difficult to find this website again, particularly if you wanted to return a few days or weeks after discovering the site. In other words, consolidation is key for memory retrieval; memories are only useful to our learning if we can retrieve them when needed. Sleep is the brain’s internal organizing process, grouping old and new information and tagging the information for retrieval in the future.
Memories can be divided into two types: episodic and procedural. Episodic memories involve specific memories of facts, events, and experiences, such as math facts, dates from history, or your previous birthday party. Procedural memories are memories of particular actions, such as riding a bike, tying your shoes, or playing a new scale on the piano. Importantly, sleep is critical for the consolidation of both types of memories, although there are differences in the brain regions that accomplish this task and in the phases of sleep that are most beneficial for each type of memory. Slow-wave sleep (SWS) which is concentrated in the first part of the night, is most strongly linked to improvements in episodic memory, whereas rapid eye movement (REM) sleep is most strongly linked to improvements in procedural memory (Diekelmann, 2010; Payne, 2011). These patterns mean that a full night’s sleep is necessary to gain the maximal benefit for both episodic and procedural memory.
The role of sleep in memory consolidation has been worked out through experiments that carefully control for the effects of time of day and the passage of time while isolating the effects of sleep. The consensus of this research is that performance on episodic and procedural memory tasks improves after episodes of sleep, compared to a matched period of wakefulness (Diekelmann, 2010). This improvement is remarkable considering that the research participants in the sleep conditions did not rehearse the information or practice the task further, they only slept. There are very few instances in which our learning and memory improves without a lot of concentrated effort, but sleep seems to be one of them.
How does the brain accomplish consolidation? Research in animal models and humans suggests that neural circuits that were active during the initial encoding of information are re-activated during consolidation. This re-activation process is believed to “stabilize” the memory. In humans and animal models, it is clear that the spatial layout and timing of neural firing that accompanies the initial learning is re-activated during sleep (Wilson, 1994; Peigneux, 2004); that is, the brain appears to repeat the initial neurological process, and “re-learn” during sleep. These results imply that our brain works in a sequenced and organized manner to consolidate memories while we are sleeping.
One controversy in the area of sleep research is whether sleep is passive or active in the consolidation process (Diekelmann, 2010). In other words, does sleep facilitate consolidation by simply preventing interference from other information while unconscious, or does sleep actively participate in the consolidation process? Although the controversy continues, evidence pointing to a new understanding of previously learned information after sleep supports the active consolidation argument. For example, many of us have experienced an “aha” moment regarding a difficult problem after a night’s sleep. These anectdotal experiences are supported by research documenting an increase in problem-solving ability after sleep compared to an equivalent time of wakefulness.
Taken together, the evidence is clear that sleep is important for memory consolidation across different forms of memory, both facts and events (like historical facts and autobiographical memories) and motor memories (playing a sport, learning a musical instrument). Although we think of sleep as a time of rest, in reality, our brain is working in a methodical, organized fashion to consolidate memories while we are sleeping. After a period of sleep, even without any additional practice, performance improves on memory tasks. The improvement in performance may even be accompanied by a new insight into a difficult problem or a discovery of new relationships between previously unconnected pieces of information. Societal pressures and technological innovations have been gradually eroding the value we place on sleep and our sleep habits. Nevertheless, the neurobiological evidence suggests that time spent sleeping is far from wasted, but instead is critical to efficient cognition. The recommendation to “sleep on it” predates much of this neurobiology, but continues to be wise advice.
For further reading:
Diekelmann, S., & Born, J. (2010). The memory function of sleep. Nature reviews. Neuroscience, 11(2), 114–26. doi:10.1038/nrn2762
Gomez, R. L., Newman-Smith, K. C., Breslin, J. H., & Bootzin, R. R. (2011). Learning, Memory, and Sleep in Children. Sleep Medicine Clinics, 6(1), 45–57. doi:10.1016/j.jsmc.2010.12.002
Payne, J. D. (2011). Learning, Memory, and Sleep in Humans. Sleep Medicine Clinics, 6(1), 15–30. doi:10.1016/j.jsmc.2010.12.005