Key takeaways:
- Awakening follows a structured sequence in the brain: activity starts in central and frontal regions and spreads toward the back.
- The transition differs by sleep stage. Non-REM awakenings show a brief surge of slow waves before faster wake activity, while REM awakenings skip slow waves and transition directly into fast activity.
- Participants reported feeling sleepier after waking from REM sleep compared to non-REM.
- Findings may inform research on insomnia, incomplete awakenings, and hyperarousal in sleep disorders.
What happens in the brain when we wake up? Researchers have uncovered a strikingly consistent pattern in how brain activity unfolds during the transition from sleep to wakefulness.
To better understand the awakening brain, researchers from the Netherlands Institute for Neuroscience and the University of Lausanne analyzed over 1,000 awakenings using high-density EEG recordings on a second-by-second basis. The study, published in Current Biology, reveals that the brain doesn’t wake up all at once. Instead, it orchestrates a precise sequence of activation.
Gradually Spreading to the Back
The researchers worked with high-density EEG data, which offers information about the time and location of brain activity. When looking at the activity progression throughout the awakening brain, they observed a clear sequence: it starts in central and frontal brain regions and gradually spreads toward the back of the brain.
Aurélie Stephan, PhD, first author, is not surprised by this sequence of events. “This progression likely reflects how signals from subcortical arousal centres (deeper in the brain) reach the cortex, with shorter paths to frontal areas and longer ones toward regions further back,” she says in a release.
REM vs non-REM
To better understand how the brain navigates waking up at any moment, the researchers specifically studied awakening patterns in REM and non-REM sleep.
- When participants awoke from non-REM sleep, their brain activity first showed a brief surge in slower sleep-like waves, immediately followed by faster activity related to wakefulness.
- When participants awoke from REM sleep, the slower waves were skipped, leading to a more direct boost in faster brain activity.
“The brain responds differently to arousing signals depending on the stage it’s in,” Stephan explains. “In non-REM sleep, neurons that connect arousal centres to the cortex alternate between states of activity and silence—a dynamic known as ‘bistability.’ As a result of this bistability, any arousing stimulus first triggers a slow wave, before transitioning to faster activity. In contrast, REM sleep does not have this bistable pattern, so the cortex immediately responds with the fast, wake-like, activity.”
Understanding Sleepiness Through Brain Waves
The researchers also investigated how sleepy a participant felt when they woke up.
While participants felt the sleepiest when awoken from REM sleep, Stephan is most intrigued by the impact of the slow waves in non-REM sleep stages.
“We found a new aspect in which slow waves can present very distinct and opposite behaviors. Some slow waves are actually acting like arousal elements—they are part of the ‘wake up!’ signal. The more these waves occur just before awakening, the more alert you tend to feel upon awakening. While the other slow waves—whether they are present before waking up or persisting after—are the reason we sometimes feel so sleepy in the first moments of the day,” Stephan says.
Stephan hopes these findings can be used for future research into sleep disorders, such as insomnia or conditions involving incomplete awakenings. “If we understand the process more, we can also better identify signs of hyperarousal in sleep disorders,” she says.
“This study provides a new perspective on the brain’s journey from sleep to wakefulness, offering a window into one of the most fundamental transitions in human consciousness.”
