Written by Brett Weiss
Sleep, a state of unconsciousness that renders an organism helpless during its course, has perplexed biologists and neuroscientists for decades. How could sleep evolve in so many organisms, and what evolutionary value does this process have? Scientists studying sleep have come up with a few hypotheses to try and explain the benefits of sleep to organisms which sleep. Most of the hypotheses relate to memory storage (consolidation) and emotional processing. The following paragraphs will provide an explanation of sleep’s relation to memory processing.
Sleep facilitates the acquisition of skills and knowledge as well allowing humans to experience themselves as coherent entities in space and time (Maier & Nissen, 2017). The ability of the central nervous system (CNS) to adapt its structure and function to a changing environment constitutes neural plasticity. Neural plasticity, the physiological correlate to learning and memory, occurs at the level of synapses. Synapses connect neurons (brain cells) to one another with electrical and chemical contact points that allow communication between neurons. Communicating neurons acquire persistently strengthened neurotransmission (long-term potentiation [LTP]) across synapses from detection of coincident (associative) activation from relevant information. Through long-term potentiation, memory formation occurs (Maier & Nissen, 2017).
Synapses from new memory traces reactivate during sleep, both during non-rapid eye movement sleep (NREMS) and rapid eye movement sleep (REMS) (Maier & Nissen, 2017). NREMS occurs during ~80% of sleep, while REMS constitutes ~15%-20% of sleep (Langille, 2019). During NREMS, people have brainwaves with high voltage, low frequency, and synchronous activity as measured with electroencephalography (EEG). Vague, disconnected, and mundane thought patterns with high parasympathetic nervous system activity (low heart rate with slowed breathing) occurs during NREMS. REMS possesses brainwaves that give wakefulness-like low-voltage, high-frequency, and de-synchronous waveforms. REMS sleep also gives vivid emotional thoughts, muscle relaxation (muscle atonia), and sympathetic nervous system dominance (elevated heart rate and breathing). REMS entails activation of the amygdala, an area involved in fear processing, along with an area thought to have importance in dream imagery called the temporo-parieto-occipital junction. REMS also involves hypoactivation of frontal lobes (frontal part of the brain) (Langille, 2019).
Acquisition of long-term memories through consolidation happens through increasing stability of memories over time. Memories begin in working memory, a state of short-term memory where preservation of memories depends upon reverberation of neural circuits (Langille, 2019). Salient memories in working memory then get stored in the hippocampus (an area toward the middle of the brain in the medial temporal lobe) through weighted plasticity. Weighted plasticity in the hippocampus occurs rapidly with limited capacity. Weighted plasticity in the hippocampus occurs temporarily, also. During weighted plasticity, memories get stored in the hippocampus in engrams, neuronal ensembles that store memories (Frankland and Bontempi, 2005). From weighted plasticity in the hippocampus, memories get consolidated to vast circuits of the neocortex (outer layer of the brain), where they become stable and resistant to interference for long-term storage (Langille, 2019). Research has provided evidence that memory consolidation to the neocortex greatly depends upon sleep, NREMS and REMS.
Learning during wakefulness favors brain activity during sleep for memory processing. Slow wave oscillations of brainwaves during NREMS improves memory processing. Systems consolidation of memories between the hippocampus and neocortex also happens during coupling of ripple waveforms during NREMS. NREMS also facilitates forgetting of less salient memories. The NREMS activity that contributes to consolidation also mediates forgetting other memories. The brainwave oscillations of NREMS that strengthen memory representations in the neocortex activate other brain mechanisms that lead to forgetting. Sharp waves occurring during NREMS reflect memory replay and trigger memory retrieval during sleep for recall and consolidation. When a memory trace gets strengthened in the neocortex upon recall, the hippocampal trace gets weakened. The consolidation of memories in the cortex and clearance from the hippocampal engram ensures that synapses of the hippocampus remain available for encoding of new experiences. This prevents wasteful and unnecessary memory copies from storage in the brain. While sharp waves drive clearance of hippocampal engrams consolidated to the cortex, slow oscillations trim memory traces in the cortex. When slow oscillations occur in the absence of memory recall, these brainwaves depress synaptic strength, which eliminates cortical memories. Hence, important memories with more salience that get recalled and reactivated during NREMS get stabilized through competition between reactivated memory traces. This competition favors the strong memories, while weak memory traces attenuate. This scenario where weak synaptic memory traces get eliminated from memory allows for removal of incidental details while extracting the “gist” of experience, improving signal to noise ratio (Tononi and Cirelli, 2014). Improving the signal to noise ratio and extracting the “gist” of experience describes the synaptic homeostasis hypothesis (Tononi and Cirelli, 2003), which says that sleep normalizes synaptic weights to a homeostasis set point through slow wave downscaling (Langille, 2019).
REMS has hippocampal brainwave oscillations called theta waves. Replay of memories during theta waves have been found to facilitate memory consolidation to the cortex, along with the maintenance of new memories in the hippocampus. The replay of newly-acquired memories during REMS theta peaks may protect new memories while they undergo systems consolidation to the cortex (Langille, 2019). REMS cortical activity differs from hippocampal theta waves in that the theta waves of the hippocampus are synchronous and slow, while the cortex shows de-synchronous and fast activity. REMS cortical activity appears to deal with memories that have already undergone consolidation. Hence, memories that have consolidated to the cortex during NREMS may reactivate during REMS for pattern completion. This reactivation of consolidated memories during REMS could function to facilitate synaptic maintenance, memory integration, and construction of new neural substrates for future learning (Tononi and Cirelli, 2014; Langille, 2019). Importantly, REMS affects new, NREMS consolidated memories for schematic integration and formation of novel connections. Formation of novel connections allows for abstraction and divergent thought (Rasch and Born, 2013; Tononi and Cirelli, 2014; Langille, 2019).
REMS theta activity of the hippocampus mediates forgetting, freeing up neural substrate for new learning. REMS theta wave ‘troughs,’ the lower voltage of the waveform, causes depotentiation whereby weak memory traces get ‘pruned’ or eliminated. Non-theta REMS activity can also lead to forgetting. Declarative memories which one may verbalize get stored in the frontal lobe. A paucity of activity in the frontal lobe is a hallmark of REMS (Nir and Tononi, 2010). Low frequency neural activity may initiate subthreshold activation of neurons along with weak pattern completion, leading to deterioration of memories stored in the frontal lobe. Stronger memory representations may exist in cooperative assemblies that would provide protection from such a mechanism (Gonzalez-Rueda et al., 2018). Thus, lack of activity in the frontal lobe (hypofrontality) during REMS could shape new long-term memories to extract the “gist” of experience while trimming away unnecessary details (Rasch and Born, 2013). Intense activity in much of the cortex during REMS has been shown to function in integrating memories into various schemas (Sterpenich et al., 2014). Integration of memories has the cost of memories losing their individuality. Integrating memories may also lead to inappropriate generalization of memories. This generally advantageous process of schematic generalization facilitates abstraction and divergent problem solving (Langille, 2019).
Emotional processing occurs during REM sleep as well. Researchers have demonstrated that REMS can cause “forgetting” of a memory’s emotional valence (van der Helm and Walker, 2011). Markers for REMS activity have been correlated with reduced amygdala (region for emotional processing) activity associated with memories (Langille, 2019).
Current research on sleep and its role in memory consolidation and emotional processing demonstrates the clear importance of sleep for overall psychological health. The two stages of sleep, NREMS and REMS, have roles in consolidating memories from the hippocampus to the cortex; and both stages of sleep possess mechanisms for ‘pruning’ memories, while REMS ‘integrates’ memories for abstraction and divergent problem solving. Attaining non-pharmacologically induced sleep is important as sleep-inducing pharmacological agents, such as benzodiazepines, have shown to inhibit synaptic plasticity and memory formation (Maier and Nissen, 2017).
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