Do you believe that sufficient sleep enhances our memory?

What is the purpose of sleep? Given our susceptibility to external hazards during sleep, significant evolutionary pressure likely exists to allocate around one-third of our lives to this unusual condition. The broad advantages of sleep for physical and mental well-being, together with the detrimental consequences of prolonged sleep deprivation, are now widely recognised. Consequently, contemporary civilisation exhibits an apparently insatiable desire to optimise sleep, whether through smart beds or various devices that monitor sleep quality from the previous night. However, the precise mechanisms via which sleep confers its advantageous effects remain unclear, continuing to perplex experts across all disciplines. To complicate the issue, sleep is not a uniform condition; rather, humans progress through various sleep stages during the night, each believed to fulfil distinct purposes.                  

While the understanding of sleep's various functions remains incomplete, one of the most fascinating benefits from a cognitive neuroscience standpoint is the enhancement of new memories, known as 'memory consolidation.' A sleep period following learning reduces the rate of forgetting compared to an equivalent duration of wakefulness.                  

Figure 1: Sleep between study ('encoding') and test ('retrieval') bolsters memory - a process called 'consolidation'. But what are the underlying mechanisms?

What are the advantages of sleep for memory retention? A straightforward explanation is that it protects the brain from incoming information that could disrupt the establishment of enduring memory traces.  Nevertheless, advancements in brain imaging indicate significantly more complex mechanisms than simple shielding from distractions. Specifically, by affixing electrodes to the scalp prior to individuals retiring for the night, sleep Electroencephalography (EEG) has demonstrated that the sleeping brain is far from inactive. Instead, it generates a captivating ensemble of electrophysiological rhythms, shown as oscillatory activity in the EEG signal. The two most significant rhythms are 'slow waves' (which contribute to the term 'Slow Wave Sleep', indicating deep sleep) and 'spindles'. Experimental research has consistently associated both cycles independently and their interplay with active memory activities during sleep. An evening of intensive learning results in increased slow waves and spindles during subsequent sleep, and experimentally augmenting slow waves with non-invasive brain stimulation appears to enhance overnight memory retention. Contemporary models suggest that slow waves and spindles represent the inherent mechanisms of the sleeping brain for transferring new memories from the hippocampus, its initial memory center, to other brain regions more appropriate for long-term retention.                    

Figure 2: In 'Targeted Memory Reactivation' protocols, experimenters play - unbeknownst to participants - reminder cues during deep sleep in an effort so bolster memory consolidation.

Recently, experimentalists have focused on memory processes during sleep and enquired whether we may enhance consolidation with slight stimulation. The concept involves providing audio or olfactory stimuli linked to prior learning content to people who are asleep. Research indicates that the rate of forgetting certain learnt material can be mitigated in this manner. Effect sizes remain rather modest; however, there exists significant potential for enhancement, such as synchronising the administration of reminder cues with endogenous sleep rhythms (slow waves or spindles) using a protocol referred to as closed-loop targeted memory reactivation (TMR).

Credits: 

Original author: Bernhard Staresina

https://www.psy.ox.ac.uk/news/read-our-blogs/sleep-research-can-sleep-improve-our-memory?ref=image

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 Biology, 28(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.