Researchers from the University of Michigan have discovered why all living beings experience alternating phases of slow-wave and rapid-eye-movement (REM) sleep. According to their study, this structure plays a crucial role in preserving and “clearing” memories. The findings were published in PLOS Computational Biology.

The team monitored the brains of sleeping mice, focusing on the activity of neurons in the hippocampus — the brain region associated with memory. They found that during slow-wave sleep, neurons involved in consolidating recent events become more active, while in REM sleep, the brain “trims” unnecessary connections to keep memories clear. Disrupting this cycle, the authors warn, could impair memory instead of strengthening it.

To test their hypothesis, the researchers used both behavioral experiments and computer modeling. They analyzed virtual neural networks made up of excitatory and inhibitory brain cells. It turned out that levels of the neurotransmitter acetylcholine — which vary depending on the sleep phase — determine which neural connections are strengthened and which are weakened.

“Slow sleep gives memories a chance to take root, while REM sleep organizes them so they don’t get jumbled. It’s like tending a bush: first you let it grow, then you trim it into shape,” explained co-author Professor Sara Aton.

Although the model currently reflects only basic processes, the scientists see it as a significant step toward understanding the mechanisms of memory and the causes of sleep disorders such as insomnia and narcolepsy. In future research, they plan to explore how the brain processes more complex memories, including emotional content and sequences over time.