When you go to sleep, your whole body relaxes, but your brain actually never rests. Increasing evidence suggests that sleep is important for several cognitive functions, one of which is memory.
Understanding the role sleep plays in enhancing memory requires a discussion of the two main stages of sleep: rapid eye movement (REM) and non-REM (NREM). During REM sleep most dreams occur, accompanied by rapid eye movements. Muscle activity is inhibited to prevent the body acting out dreams. Therefore, the REM sleep can be described as “an activated brain in a paralyzed body.”
The brain is less active in NREM sleep. NREM can be further divided into four stages. Stage 3 and stage 4, also known as slow-wave sleep (SWS), are hypothesized to be linked to memory formation. Most adults usually enter sleep through NREM stage, heading from stage 1 to stage 4 NREM, and finally enter REM sleep. A cycle of NREM and REM usually last around 90 to 110 minutes, with NREM dominating the early cycles at night and REM dominating the later cycles at night or in the morning.
Sleep can promote the consolidation of memory. The consolidation process incorporates knowledge learned during the day into our preexisting network of knowledge and strengthens the neural representation of important information. There are two hypotheses regarding the memory function of sleep: the synaptic homeostasis hypothesis and the active system consolidation.
The synaptic homeostasis hypothesis was developed by Giulio Tononi and Chiara Cirelli of the University of Wisconsin-Madison. They propose a downscaling of synaptic strength during sleep, in which the strength of all the synapses in the brain decreases. In other words, the strength of the synapses that are weakly activated during the day decrease to below threshold at night, corresponding to forgetting weak memories. For example, we might forget something we read once in a textbook, because the memory is weakly encoded. Lasting memories, by contrast, also have reduced synaptic strength, but the synaptic connection itself remains strong. In this sense, reducing the signal-to-noise ratio promotes the preservation of relevant memory. However, the hypothesis fails to explain that sometimes weak memory can also benefit from sleep. A recent paper suggests “a prioritization of weakly learned information early in a sleep period”, which means weak memories are not always erased immediately. Despite its limitations, the synaptic homeostasis hypothesis suggests an important function of sleep, which is “clearing the cache” to save room for the encoding of future memory.
The strengthening of one memory thus might jeopardize the preservation of the other memory.
Active system consolidation, is a theory proposed by German researchers Jan Born and Ines Wilhelm. They posit that newly coded memories and old memories are stored in different brain regions. During waking hours, information is stored in the hippocampus, and this new knowledge must be transferred into the “preexisting knowledge network” in neocortex. According to this hypothesis, two copies of the same piece of new memory are encoded during the day, one that is temporarily stored in the hippocampus, and another that is stored for the long term in the neocortex. However, the neurons in the neocortex that represent the memory are weakly connected to each other. During the slow-wave stage of sleep, the neural network of the new memory in hippocampus replays itself, activating the corresponding neural representation in the neocortex and strengthening the neocortical network. In this way, the short-term memory can be transferred to long-term storage.
However, this new memory disturbs the pre-existing network. For example, a neuron might have been previously involved in one network representing an old memory. The addition of a new memory gives that same neuron a new role in a different network, which means the neuron is now involved in the memory of two pieces of information. The strengthening of one memory thus might jeopardize the preservation of the other memory. To solve this problem, during the REM stage following slow-wave sleep, the synapses consolidate and stabilize themselves so that the neural presentation of the new memory and the connection between the new memory and surrounding neurons are strengthened. New synapses between those neurons are created, and more receptors are synthesized and are transported to the surface of these synapses. As a result, the neuron can successfully represent two memories in two networks, and new memories are thus able to be incorporated without overwriting the old ones.
There is still much to be known about how sleep influences memory. But as far as we know, the brain is doing a lot more during sleep than we previously assumed.