Neuroscientists studying the shifts between sleep and awareness are finding many liminal states, which could help explain the disorders that can result when sleep transitions go wrong. New research strives to understand what happens in the brain at the transitions between sleeping and being awake. The pillow is cold against your cheek. Your upstairs neighbor creaks across the ceiling. You close your eyes; shadows and light dance over your vision. A cat sniffs at a piece of cheese. Dots fall into a lake. All this feels very normal and fine, even though you don’t own a cat and you’re nowhere near a lake. You’ve started your journey into sleep, the cryptic state that you and most other animals need in some form to survive. Sleep refreshes the brain and body in ways we don’t fully understand: repairing tissues, clearing out toxins, and solidifying memories. But as anyone who has experienced insomnia can attest, entering that state isn’t physiologically or psychologically simple. To fall asleep, “everything has to change,” says Adam Horowitz, a research affiliate in sleep science at MIT. The flow of blood to the brain slows down, and the circulation of cerebrospinal fluid speeds up. Neurons release neurotransmitters that shift the brain’s chemistry, and they start to behave differently, firing more in sync with one another. Mental images float in and out. Thoughts begin to warp. It’s still largely mysterious how the brain manages to move between these states safely and efficiently. But studies targeting transitions both into and out of sleep are starting to unravel the neurobiological underpinnings of these in-between states, yielding an understanding that could explain how sleep disorders, such as insomnia or sleep paralysis, can result when things go awry. Sleep has been traditionally thought of as an all-or-nothing phenomenon, Lewis says. You’re either awake or asleep. But the new findings are showing that it’s “much more of a spectrum than it is a category.
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