Falling asleep isn’t a gradual process – it happens all of a sudden

The brain is not gradual fall asleep. Instead, it reaches a tipping point where it quickly transitions from wakefulness to sleep in a matter of minutes—a discovery that could improve our understanding and treatment of sleep disorders such as insomnia.

“Although sleep is so important to our lives, how the brain falls asleep remains a mystery,” says Nir Grossman at Imperial College London. It is widely believed that this is a gradual process in which the brain gradually transitions from wakefulness to sleep. But the evidence to support this is limited.

Grossman and his colleagues have developed a new framework for studying how the brain behaves when we fall asleep using electroencephalography (EEG) data. This test, which records the electrical activity of the brain, determines the stages of sleep and wakefulness. The team modeled 47 EEG signals in an abstract mathematical space, where each data point had coordinates, as if it were a point on a map. This allowed the team to chart brain activity during wakefulness and track it as they move toward the so-called sleep onset zone, where brain activity corresponds to the second stage of non-rapid eye movement (NREM) sleep.

“We can now take a person, measure brain activity, and tell every second how far away they are from falling asleep, every moment, with an accuracy that was not possible before,” says Grossman.

They applied this approach to EEG data collected from more than 1,000 people during sleep, measuring the distance between brain activity and the onset of sleep. On average, this distance remained virtually unchanged until 10 minutes before bedtime, and then dropped sharply in the last few minutes. This tipping point, which occurs an average of 4.5 minutes before bedtime, is the exact moment when the brain switches between wakefulness and sleep, scientists say. Junheng Lialso at Imperial College London. “[This] This is the point of no return,” he says.

These results suggest that the transition from wakefulness to sleep “is not a gradual progression. It is an abrupt, radical change that occurs in the last few minutes,” Grossman says. So the way we describe falling asleep—usually as “falling”—is largely a reflection of what's going on in the brain. “It's almost evidence of a sense of transition into another state,” Grossman says.

The team then collected EEG data from a separate group of 36 people, monitoring each participant's sleep for about a week. Using a portion of these nights, they were able to predict when participants would fall asleep within a minute of the actual moment.

“What this tells me is that, although people vary greatly, each person may have their own sleep path that they tend to repeat night after night,” says Laura Lewis at the Massachusetts Institute of Technology. But it's unclear whether this behavior pattern will change under other circumstances, such as sleeping in a new place, she says.

This theory also doesn't reveal the brain mechanisms that control the transition to sleep, Lee said. But this could help us do that in the future,” Lewis says. “As sleep set in, we had a really hard time finding that moment,” she says. “If we knew when this happened, we could start asking what brain region or circuitry causes someone to fall asleep?” By understanding the dynamics of this transition, we may also be able to determine how they differ in people with insomnia, potentially leading to new treatments for the condition, she says.