Coordinated slow waves of activity across brainstem neuron populations appear to determine when the brain transitions into rapid-eye-movement (REM) sleep, according to research published in Nature Neuroscience. The findings suggest this sleep phase emerges from distributed network activity rather than a single neural switch.
Researchers at the University of Pennsylvania and the Champalimaud Foundation monitored the brains of sleeping mice to understand how the brainstem coordinates the shift from non-REM (NREM) to REM. Using high-density Neuropixels probes, they simultaneously recorded the activity of hundreds of brainstem neurons during sleep.
The team identified two broad activity patterns affecting various neuron populations. During NREM, they observed slow waves of population-wide activity gradually building up before the transition to the REM phase. These dynamics could predict when the brain was more likely to enter dream sleep.
Senior author Franz Weber explained that rather than focusing on specific brain areas that promote or suppress this state, the team examined coordinated activity across larger brainstem networks. They identified two distinct neuron groups showing opposite activity patterns, one active during the REM phase, the other inhibited.
The researchers also combined recordings with optogenetic experiments, stimulating specific neurons to test whether the ongoing brainstem network state influences whether such stimulation successfully triggers the transition.
The findings may help researchers understand disruptions to dream sleep associated with various neurological and psychiatric disorders. This sleep phase has been linked to dreaming, emotional processing, memory consolidation, and brain development, though its core functions remain incompletely understood.
Source: Medical Xpress / University of Pennsylvania, Champalimaud Foundation (Nature Neuroscience, 2026)
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