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

The brain does not gradually fall asleep. Instead, it reaches a critical point at which it rapidly transitions from wakefulness to sleep within minutes – a finding that could improve our understanding and treatment of sleep disorders like insomnia.

“Although sleep is so fundamental to our lives, how the brain falls asleep remains a mystery,” says Nir Grossman of Imperial College London. It is generally thought to be a gradual process in which the brain gradually transitions from wakefulness to sleep. But the evidence to support this claim is limited.

Grossman and colleagues designed a new framework to study the brain’s behavior as we fall asleep using electroencephalography (EEG) data. This test, which records electrical activity in the brain, indicates the phases 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 trace brain activity during wakefulness and track it as it moves toward what they call the sleep onset zone, where brain activity corresponds to the second stage of non-rapid eye movement (NREM) sleep.

“We can now take an individual, measure their brain activity and, in each second, tell how close they are to falling asleep, in each moment, with a precision that was not possible before,” Grossman says.

They applied this approach to EEG data collected from more than 1,000 people as they fell asleep, measuring the distance between brain activity and the onset of sleep. On average, this distance remained largely unchanged until 10 minutes before bedtime, then decreased sharply in the last few minutes. This tipping point – which occurs on average 4.5 minutes before sleep – is the exact moment when the brain switches between wakefulness and sleep, says Junheng Li, also at Imperial College London. “[This] This is the point of no return,” he says.

These findings suggest that the transition from wakefulness to sleep “is not a gradual progression. It is an abrupt, drastic change that occurs over the last few minutes,” says Grossman. So the way we describe entering sleep – usually as a “fall” – largely reflects what is happening in the brain. “It’s almost proof of that feeling of falling into a different 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 subset of these nights, they were able to predict when participants would fall asleep within a minute of the actual time.

“What this suggests to me is that even though people are very different from each other, each individual may have their own path to sleep that they tend to repeat night after night,” says Laura Lewis of the Massachusetts Institute of Technology. But it’s unclear whether this trend would change under different circumstances, such as sleeping in a new place, she said.

This framework also doesn’t reveal the brain mechanisms that lead to the transition to sleep, Li says. But it could help us do that in the future, Lewis believes. “With falling asleep, it was very difficult for us to find that moment,” she says. “If we knew when this was happening, we could then start to ask ourselves, what is the brain region or circuit that causes someone to fall asleep?” By understanding the dynamics of this transition, we may also be able to identify how they differ in people with insomnia, which could lead to new treatments for this condition, she says.