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For World Sleep Day 2022, Bernhard Staresina discusses research into the effects of sleep on memory consolidation.

Woman asleep at her desk with lamp on and book in front of her

Why do we sleep? Considering our vulnerability to external threats while asleep, there must be great evolutionary pressure to spend roughly a third of our lives in this peculiar state. Indeed, the global benefits of sleep for physical and mental health, as well as the debilitating effects of chronic sleep deficit, are well established by now. Consequently, today’s society has a seemingly insatiable appetite to optimise sleep – be it via smart mattresses or other gadgets telling us how well we’ve slept last night. Yet, how exactly sleep exerts its beneficial effects has remained elusive and continues to puzzle researchers from various fields. To confuse matters further, sleep is not a single monolithic state, but throughout the night we cycle through different sleep stages, each thought to serve potentially different functions.

Although deciphering the many roles of sleep is still a work in progress, one of the most intriguing effects of

Figure 1: Sleep between study ('encoding') and test ('retrieval') bolsters memory - a process called 'consolidation'. But what are the underlying mechanisms?Figure 1: Sleep between study ('encoding') and test ('retrieval') bolsters memory - a process called 'consolidation'. But what are the underlying mechanisms? sleep from a cognitive neuroscientist’s perspective is the strengthening of new memories – a process called ‘memory consolidation’. Specifically, a period of sleep after learning slows down forgetting compared to the same time spent awake.

Why would sleep be beneficial for memory? A simple explanation might be that it shelters the brain from incoming information that would otherwise interfere with the process of laying down lasting memory traces.  However, advances in brain imaging point to much more intricate processes than mere protection from distractors. In particular, by sticking electrodes to the scalp before letting participants go to bed, sleep Electroencephalography (EEG) has revealed that the sleeping brain is everything but switched off. Instead, it produces a mesmerising concert of electrophysiological rhythms, expressed as oscillatory activity in the EEG signal. The two most prominent among these rhythms are ‘slow waves’ (giving rise to the name ‘Slow Wave Sleep’, which denotes deep sleep) and ‘spindles’. Experimental work has consistently linked both rhythms on their own as well as their interaction to active memory processes during sleep. For example, an evening of heavy learning brings about greater levels of slow waves and spindles during subsequent sleep, and experimentally enhancing slow waves via non-invasive brain stimulation seems to bolster overnight memory retention. Current models propose that slow waves and spindles reflect the sleeping brain’s internal machinery for transporting new memories out of the brain’s early memory hub (the hippocampus), ‘parking’ them in other brain regions better suited for long-term storage.  

More recently, experimentalists have homed in on these memory processes during sleep and asked whether we can

Figure 2: In 'Targeted Memory Reactivation' protocols, experimenters play - unbeknownst to participants - reminder cues during deep sleep in an effort so bolster memory consolidation.Figure 2: In 'Targeted Memory Reactivation' protocols, experimenters play - unbeknownst to participants - reminder cues during deep sleep in an effort so bolster memory consolidation. give them a little nudge to boost consolidation. In brief, the idea is to deliver auditory or olfactory cues associated with previous learning material to sleeping participants. Research has shown that forgetting specific learning content can indeed be slowed down in this fashion. Effect sizes still tend to be rather modest, but there is ample room for improvement, for instance by timing the delivery of reminder cues to be in sync with endogenous sleep rhythms mentioned above (slow waves or spindles) – a protocol known as closed-loop targeted memory reactivation (TMR).

At Oxford, we are in the final stages of establishing a dedicated sleep laboratory in the Oxford Centre forFigure 3: OHBA nights - a new sleep laboratory at the Oxford Centre for Human Brain Activity.Figure 3: OHBA nights - a new sleep laboratory at the Oxford Centre for Human Brain Activity. Human Brain Activity (OHBA). The building houses a state-of-the-art Magnetic Resonance Imaging (MRI) scanner as well as a Magnetoencephaloghy (MEG) scanner and brain stimulation suits. It is thus the ideal site to examine the role of sleep for memory from all possible angles, including the effects of experimentally enhancing endogenous sleep rhythms. Funded by a European Research Council (ERC) Consolidator grant, an exciting new avenue will be to link the emergence of particular sleep rhythms to structural brain changes, i.e., the physical substrate of learning and memory. For more information, visit www.staresinalab.com.

World Sleep Day is a welcome reminder of the exciting times ahead for sleep research. Erroneously considered a waste of time in the past, there is now an ever-increasing appreciation for sleep as a window of opportunity – an opportunity not only to improve physical and mental health, but also to bolster memory and combat its decline in natural ageing or neurodegenerative diseases.

 

Further reading:

Cairney, S.A., Guttesen, A., El Marj, N. and Staresina, B.P., 2018. Memory consolidation is linked to spindle-mediated information processing during sleep. Current Biology28(6), pp.948-954.

Rasch, B. and Born, J., 2013. About sleep's role in memory. Physiological reviews.

Staresina, B.P., Bergmann, T.O., Bonnefond, M., Van Der Meij, R., Jensen, O., Deuker, L., Elger, C.E., Axmacher, N. and Fell, J., 2015. Hierarchical nesting of slow oscillations, spindles and ripples in the human hippocampus during sleep. Nature neuroscience, 18(11), pp.1679-1686.

Walker, M. (2017). Why we sleep: Unlocking the power of sleep and dreams. Simon and Schuster.