Why Do We Remember Better When We Sleep?

 

    Although there is still much to be discovered about why humans need sleep to survive, one piece of the puzzle we are sure about is that our memories are largely dependent on it. In their article “Mechanisms of systems memory consolidation during sleep,” Klinzing and his colleagues discovered that sleep not only enhances our ability to directly recall memories, but also transforms them in a way that allows us to abstract them for more general information.

    When we are born, we don’t yet have consistent sleep patterns because our bodies have such a high metabolic demand (as well as low nutritional storage abilities) as babies that we need to rapidly go back and forth between sleeping and feeding (Cavanaugh et al., 10). However, as we mature, we are able to better sustain nutrition and we have more to gain from circadian control because the patterns and longer sleep duration allow for more sophisticated brain functioning. In their article “Developmental emergence of sleep rhythms enables long-term memory capabilities in Drosophila,” Cavanaugh and his colleagues study this transition from random sleep patterns to circadian rhythms as well as its influence on long-term memory (LTM) using drosophila. They found that sleep rhythms and circadian control were not necessary for short-term memory, but were necessary for LTM. They also found that sleep deprivation disrupted the formation of LTMs.

    The Cavanaugh et al. study explains how LTM facilitation is only possible after circadian control develops, and the Klinzing et al. study expands on this idea by looking into the actual mechanisms by which we consolidate memories. One important way we do this is through active systems consolidation, which is how we transform newly encoded representations into stable LTMs in the hippocampus-dependent episodic memory system (Klinzing et al., 1). A particular focus of their study was the thalamus and its ability to regulate consolidation of specific memories during the sleep cycle. They exposed mice to a novel stimulus and observed that a parallel orientation-specific response occurred in the visual cortex, but only after the mice had slept. At first exposure, sensory neurons relayed the information to the thalamus so that it could be quickly encoded, then the thalamus told the neocortex to form a corresponding memory as the mice slept. During sleep, sleep spindles (which are electrographic landmarks for the transition from waking to sleeping) and slow oscillation activity are able to coordinate transmission of the memory from the thalamus to the neocortex. This shows that sleep is necessary for sensory memories to consolidate and be transmitted from the thalamus to the neocortex, where it can become an LTM.

    Further research is needed on the thalamus in particular, especially to see if the results Klinzing et al. found can be replicated for other sensory modalities as well as more complex stimuli. It would also be beneficial to more deeply study the effects of sleep deprivation on LTM development. The more we learn about the how, where, and why of LTM consolidation, the closer we are to understanding the many mysteries of the functions of sleep in humans.

Resources:

Amy R. Poe, Lucy Zhu, Patrick D. McClanahan, Milan Szuperak, Ron C. Anafi, Andreas S. Thum, Daniel J. Cavanaugh, Matthew S. Kayser bioRxiv 2022.02.03.479025,https://doi.org/10.1101/2022.02.03.479025

Klinzing, J.G., Niethard, N. & Born, J. Mechanisms of systems memory consolidation during sleep. Nat Neurosci 22, 1598–1610 (2019). https://doi.org/10.1038/s41593-019-0467-3