Lights-out in your brain: Sleeping one region at a time

May 12th, 2026

Written by Sophia Rueda

Have you ever tried finishing an important task late at night and felt like parts of your brain have already clocked out? For example, as I sit here writing this at 11pm, I have the suspicious feeling that my brain, or at least parts of it, have gone to sleep without me. As it turns out, that feeling is actually a lot closer to the truth than you might think. 

We often imagine sleep as an on-off switch that shuts down our whole brain at once, like a citywide blackout. But for our brains, sleep acts more like a city dimming its lights, neighborhood by neighborhood. We call this local sleep, which is the process of only a fraction of brain regions sleeping at a given time while others remain awake. In other words, ‘local’ refers to only certain regions being asleep at once. This process occurs throughout the entire time we are asleep, with some regions turning off their lights earlier than others and different areas becoming active again at different points in the night.


What does local sleep look like in the brain?

One example of where we can see local sleep is during sleep onset, the moment we begin to fall asleep. To track which brain regions are drifting into sleep at what time, scientists use tools to measure human brain activity. These tools include electroencephalography (EEG), which measures electrical patterns of brain activity, and functional magnetic resonance imaging (fMRI)2, which measures blood flow to active brain areas. Think of it this way: fMRI can tell us which city blocks still have their lights on, while EEG tracks the precise moment those lights begin to flicker and dim. 

Researchers found that during the transition into sleep, the lights are first turned off at the center of the city (the regions deep in the brain), such as the hypothalamus, which controls many basic drives like hunger and thirst.1,3 These center-city regions then communicate with other parts of the brain using chemicals that act as “stop” signals, called inhibitory neurotransmitters. Their role is to quiet down activity in other regions, encouraging them to turn off their lights and transition into sleep. The thalamus, which relays motor and sensory information to other parts of our brain, dims its lights and slows down its activity early in the night, encouraging areas like the hippocampus, which is in charge of our memory and learning, to transition into sleep. A few minutes later, the cortex, which controls higher level thinking, begins turning off its lights. But that doesn’t happen uniformly either. The frontal regions of the cortex, which control our ability to plan and make decisions, fall asleep before the regions in the back, which are responsible for processing vision and other sensory information.1,4

This staggered neighborhood-by-neighborhood transition does not just happen during sleep onset. Local sleep continues throughout the night as we move through different stages of sleep, with brain regions shifting between lighting up and dimming at different times. For example, we can see this when our brain moves between NREM (non-rapid eye movement), which is deep sleep focused on memory consolidation and recovery, and REM, (rapid eye movement), the stage most strongly associated with dreaming. During REM sleep, the visual regions of the brain become active again, which explains the vivid visual imagery that often characterizes dreaming (you can read more about our dreaming brain here).4 So even though local sleep happen throughout the night, we’ll continue using sleep onset as our main example of this process, because in the few minutes it takes us to drift into sleep, we can see local sleep in action.

The hypnagogic state 

The staggered timing of sleep onset creates an interesting in-between state of consciousness called hypnagogia,5 where we become drowsy and begin the transition from wakefulness into sleep. You can blame hypnagogia for waking up and realizing you only remember fragments of the TV show you watched last night and will need to rewatch the whole thing. It’s also the reason why you can still hear someone say your name as you’re drifting in and out of awareness. When we are in this state, our deeper brain regions have turned off their lights, while our sensory regions remain active and attentive to activity in the environment, like sounds. As a result, we can experience the world around us but we do not store the memories of it.6 

Why do we fall asleep this way?

Scientists are still not sure exactly why we transition into sleep this way, but one theory involves the idea of sleep pressure, which is the concept that the biological urge to sleep builds the longer we stay awake.7 This pressure grows with the physical and mental intensity of our day as the chemical adenosine accumulates in the brain and promotes sleepiness.8 It could be that the parts of the brain that have worked the hardest while we are awake may have the highest sleep pressure and need to rest first. If true, this would explain why the frontal cortex regions, which are responsible for many complex functions like planning and problem solving, may need to turn their lights off before sensory regions.

Is local sleep a secret superpower?

It is not well understood whether this process of gradual descent into sleep occurs because it is beneficial to us, or just a random byproduct of evolution. On the one hand, research has discovered that hypnagogia can function as a “creative sweet spot” for problem solving. In one study, participants attempted to solve complex math problems without being told that there is a hidden rule which would allow them to solve the problem almost instantly. Those who attempted to solve the problems after being awoken from this hypnagogic, sleep-onset state, were three times more likely to find the hidden rule than those who were awoken during deeper sleep stages.9 On the other hand, during hypnagogia people also report having unpleasant experiences including sleep paralysis and hypnagogic hallucinations, which are vivid dream-like experiences such as hearing voices, seeing shadows, or feeling like you’re falling.10 Future studies will continue exploring the benefits and potential drawbacks of local sleep and why humans have evolved this complex sleep pattern. 

For humans, sleep isn’t a global blackout. Instead it’s more like a city gradually turning off its lights, neighborhood by neighborhood. While not entirely understood, this process shows us how complex and elegant our brains are, finding ways to balance rest while maintaining enough awareness to keep us safe as we fall asleep. So next time you are up late working and feel like part of your brain has clocked out before the rest of you, maybe try looking at a really complex math problem. 

References

  1. Song, C., Boly, M., Tagliazucchi, E., Laufs, H., & Tononi, G. (2022). fMRI spectral signatures of sleep. Proceedings of the National Academy of Sciences, 119(30), e2016732119. https://doi.org/10.1073/pnas.2016732119
  2. Manjarrez, A. (2022). Which Neurons Go to Sleep First in Humans? fMRI Can Tell. The Scientist. Retrieved May 4, 2026, from https://www.the-scientist.com/which-neurons-go-to-sleep-first-in-humans-fmri-can-tell-70340
  3. Marzano, C., Moroni, F., Gorgoni, M., Nobili, L., Ferrara, M., & De Gennaro, L. (2013). How we fall asleep: Regional and temporal differences in electroencephalographic synchronization at sleep onset. Sleep Medicine, 14(11), 1112–1122. https://doi.org/10.1016/j.sleep.2013.05.021
  4. Peter-Derex, L., von Ellenrieder, N., van Rosmalen, F., Hall, J., Dubeau, F., Gotman, J., & Frauscher, B. (n.d.). Regional variability in intracerebral properties of NREM to REM sleep transitions in humans. Proceedings of the National Academy of Sciences of the United States of America, 120(26), e2300387120. https://doi.org/10.1073/pnas.2300387120
  5. Ghibellini, R., & Meier, B. (2023). The hypnagogic state: A brief update. Journal of Sleep Research, 32(1), e13719. https://doi.org/10.1111/jsr.13719
  6. Saplakoglu, Y. (2025, October 17). How the Brain Moves From Waking Life to Sleep (and Back Again). Quanta Magazine. https://www.quantamagazine.org/how-the-brain-moves-from-waking-life-to-sleep-and-back-again-20251017/
  7. D’Ambrosio, S., Castelnovo, A., Guglielmi, O., Nobili, L., Sarasso, S., & Garbarino, S. (2019). Sleepiness as a Local Phenomenon. Frontiers in Neuroscience, 13. https://doi.org/10.3389/fnins.2019.01086
  8. Reichert, C. F., Deboer, T., & Landolt, H. (2022). Adenosine, caffeine, and sleep–wake regulation: State of the science and perspectives. Journal of Sleep Research, 31(4), e13597. https://doi.org/10.1111/jsr.13597
  9. Lacaux, C., Andrillon, T., Bastoul, C., Idir, Y., Fonteix-Galet, A., Arnulf, I., & Oudiette, D. (n.d.). Sleep onset is a creative sweet spot. Science Advances, 7(50), eabj5866. https://doi.org/10.1126/sciadv.abj5866
  10. Foffani, G. (2023). To be or not to be hallucinating: Implications of hypnagogic/hypnopompic experiences and lucid dreaming for brain disorders. PNAS Nexus, 3(1), pgad442. https://doi.org/10.1093/pnasnexus/pgad442

Perplexity powered by Grok 4.1 was used to help reword some sentences, highlight technical language, check grammar, and to brainstorm title ideas.

Cover photo by Gerd Altmann from Pixabay.

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