It’s no secret that the human body functions better with regular hours of sleep. As we continue to study the science behind why this is, we discover more and more things that sleep does for our bodies. In a 2019 study funded by the National Institute of Health titled “REM sleep-active MCH neurons are involved in forgetting hippocampus-dependent memories”, scientists in the Yamanaka lab of Nagoya University, Japan, uncovered a link between the activation of cells containing melanin concentrating hormone (MCH) during REM sleep and long term memory. A recent study by Dr. Daniel Cavanaugh and colleagues titled “Developmental emergence of sleep rhythms enables long-term memory capabilities in Drosophila” describes the development of a circadian rhythm in drosophila and how that relates to long term memory storage. In both articles, despite their different goals and approaches, it is determined that clock control of sleep in the body promoting deep sleep is necessary for the creation and storage of long term memories.
The research conducted in the Yamanaka lab did not adhere to their initial hypothesis, that MCH assist in the storage of new memories. In fact, their results showed the opposite. When the MCH cells were deactivated during sleep, the mice on which the experiment was ran showed improvements on memory tests, which suggested that cells containing MCH are used to inhibit the storage of memories rather than store them. The researchers in the lab speculate that this has to do with the relationship between REM sleep and dreams. They hypothesize that because much of dreaming occurs during REM sleep, the MCH cells are used to stop the brain from encoding dreams, which are largely irrelevant to real life, into our long term memory. Curious to see if this effect of turning off MCH cells improved memory while not in deep sleep, the researchers tested the mice with the cells turned off both while awake and in other stages of sleep. The results of these tests showed no improvement in performance on memory testing, further affirming the relationship between deep sleep and long term memory formation.
The research done by Dr. Cavanaugh and colleagues sought to investigate the relationship between the development of circadian rhythm and periods of growth, and in their investigation discovered how the development of deep sleep in drosophila allows for long term memory to be created. Dr. Cavanaugh’s lab pinpointed the development of circadian rhythm in drosophila larva to between instar larvae stage two and stage three. Initially, they were curious if circadian rhythm was required for short term memory, but they found no significant difference in the performance on memory tests. When testing long term memory however, they discovered that the larvae that had developed deep sleep were capable of storing information on odor preference tests long term, whereas the larvae that had not developed this could not retain this information.
While these two sets of researchers sought to investigate different aspects of the sleep cycle and its effects, they both happened to find a strong connection between deep sleep and the function of long term memory. Both of these studies begin to investigate the relationship of the circadian rhythm and long term memory, which has broad implications on understanding under what conditions humans will best perform. Additionally, better understanding what exactly is involved in the creation of long term memories may also help us to understand how to best treat patients with memory deficits or degenerations like Alzheimer’s disease.
References:
Poe, A. R., Zhu, L., McClanahan, P. D., Szuperak, M., Anafi, R. C., Thum, A. S., Cavanaugh, D. J., & Kayser, M. S. (2022, January 1). Developmental emergence of sleep rhythms enables long-term memory capabilities in drosophila. bioRxiv. https://www.biorxiv.org/content/10.1101/2022.02.03.479025v1
Thomas, C. G. (2019, September 19). The brain may actively forget during Dream sleep. National Institutes of Health. https://www.nih.gov/news-events/news-releases/brain-may-actively-forget-during-dream-sleep
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