Half asleep—different brain areas can be in different sleep stages at the same time

What is it about?

Although we usually think of sleep as a binary choice—we are or we are not asleep—it has long been known that there is structure within sleep, where the brain cycles between different sleep states, such as N2, N3, and REM sleep. To test whether these sleep states are always brain-wide, we compared neural activity that was recorded simultaneously in the hippocampus (the learning and memory center of the brain) and the frontal neocortex (the thought/judgement/action brain center) during the night. Across participants, these two brain regions were often—on average 30% of the time—in entirely different sleep states at the same time, including periods when the hippocampus was asleep but the cortex was awake. These findings mean that sleep is not a global brain/body state, but rather sleep can be fine-tuned, suggesting that the amount and type of sleep may be directed by the different needs of different brain regions.

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Photo by Lidiia Nemyrova on Unsplash

Photo by Lidiia Nemyrova on Unsplash

Why is it important?

This finding has significant implications for our understanding of the functions and orchestrators of sleep. To date, most sleep research in humans has relied exclusively on electrical signatures of sleep that can be recorded from the cortical surface, which means they may have missed periods of hippocampal sleep. This may explain differences in study outcomes which measure sleep from different areas. Non-simultaneous sleep states across the brain should also be taken into account in considering reports of insomnia and non-restorative sleep, and could play a role in our understanding of lucid dreaming and why some dreams are remembered. Future work to understand both what drives asynchronous sleep states and how these states affect the efficiency of sleep have the potential to reshape how we think about the role of sleep and drive a new type of therapeutics for sleep disorders.