What happens when we sleep?

In 2017, an article was published in the New York Times titled, “The Purpose of Sleep? To Forget, Scientists Say”. In this article, Carl Zimmer summarizes the work of multiple scientists and their findings on the purpose of sleep. He presented previous theories on why humans sleep, including to save energy, hide from predators, and clear away the brain’s cellular waste. He then discussed the work of Giulio Tononi and Chiara Cirelli, biologists from the University of Wisconsin-Madison. These two scientists proposed that while sleeping, human brains prune synapses to get rid of the noise and only keep what is important. While these two researchers found indirect evidence to support their hypothesis, they were unable to find a direct link between sleep and synapse pruning, what they called the synaptic homeostasis hypothesis.

It was Graham H. Diering, a postdoctoral researcher at Johns Hopkins University, who was able to find some more definitive evidence. He, along with his team, studied the synaptic homeostasis hypothesis by studying proteins in the brains of mice. In one of his studies, he was able to create a window that allowed for researchers to see what was happening in live mice brains. They found that the number of surface proteins in mice brains dropped while they slept, and this was correlated with shrinking synapses. He was able to isolate a protein, Homer1A, as the pruning mechanism while mice slept.

Diering and his team also looked at how Homer1A affected learning. He tested two groups of mice, one with Homer1A and one without, by shocking them in a specific chamber. The next day, he brought both of the groups back to the chamber and both groups froze, remembering the shock they received earlier. He then moved the groups to different chambers and the mice without Homer1A froze again, while the normal mice explored the chamber. This suggests that the mice without Homer1A were unable to narrow down their memories to the particular chamber where they had received the shock.

Synaptic homeostasis hypothesis is still not definitively proven, however there seems to be some promising research on this topic. I am excited to see where this research goes in the future and what this synaptic pruning means for humans and what we remember after sleep.

https://www.nytimes.com/2017/02/02/science/sleep-memory-brain-forgetting.html

https://science.sciencemag.org/content/355/6324/511