Researchers have discovered that the brain does not gradually fall asleep but instead undergoes a rapid transition from wakefulness to sleep in just minutes. This tipping point, identified through EEG data from over 1,000 people, challenges the long-held view of sleep onset as an incremental process. The findings could enhance understanding and treatment of sleep disorders like insomnia.
For years, scientists believed the brain eased into sleep incrementally, steadily shifting from wakefulness to rest. However, a new study led by Nir Grossman at Imperial College London upends this idea. Using electroencephalography (EEG) to record brain electrical activity, the team analyzed data from more than 1,000 individuals as they fell asleep.
The researchers modeled brain activity in an abstract mathematical space, plotting it like points on a map to track proximity to the 'sleep-onset zone'—corresponding to the second stage of non-rapid eye movement (NREM) sleep. They found that the distance from wakefulness to this zone remained stable until about 10 minutes before sleep, then plummeted abruptly in the final minutes. The critical tipping point, occurring on average 4.5 minutes before sleep, marks the irreversible switch, according to Junheng Li, also at Imperial College London. "[This] is the point of no return," he says.
"Although sleep is so fundamental to our life, how the brain falls asleep has been a mystery," Grossman notes. The abrupt change aligns with the common sensation of 'falling' asleep. "It’s almost evidence of this sensation of falling into a different state," he adds.
In a follow-up with 36 participants monitored over a week, the model predicted sleep onset within one minute on select nights. Laura Lewis at the Massachusetts Institute of Technology comments that individuals may follow repeatable paths to sleep, though these could vary with circumstances like unfamiliar environments. While the framework does not yet reveal underlying brain mechanisms, it offers a precise tool to measure sleep proximity second by second, potentially aiding research into insomnia and new therapies.
The study appears in Nature Neuroscience (DOI: 10.1038/s41593-025-02091-1).