Researchers have identified, for the first time, the precise moment the human brain transitions from wakefulness into sleep, demonstrating that falling asleep is an abrupt shift rather than a gradual drift.
The findings, published in Nature Neuroscience, reveal a measurable “tipping point” in brain activity and offer a new framework for diagnosing sleep disorders, monitoring anesthesia, and assessing brain health in ageing and neurodegenerative disease.
The study, led by the UK Dementia Research Institute (UK DRI) with core funding from the Medical Research Council (MRC), analysed electroencephalogram (EEG) recordings from more than 1,000 people who wore electrodes overnight. Using a new computational approach, the team mapped each individual’s progression through multi-dimensional brain-activity space as they moved from bedtime toward sleep.
No matter how long participants lay in bed before losing consciousness, the researchers found that the transition itself happened abruptly in the final minutes. The pattern matched a “bifurcation” – a dynamic in which a system gradually changes until it suddenly snaps into a new state, similar to a stick bending until it breaks. The researchers note that this also mirrors the sensation of “falling asleep.”
In a separate multi-night experiment, each participant showed a consistent, personalised point in this multi-dimensional space at which sleep began.
Armed with these insights, the team demonstrated that they could predict, with 98% accuracy, the second-by-second progression into sleep for each participant. Until now, sleep onset has typically been determined using subjective EEG annotations or indirect physiological measures such as heart rate, breathing, or movement.
“This new approach is the first time scientists have been able to objectively infer the precise momentary progression and exact point at which the brain falls asleep,” the authors report.
The research was carried out by scientists at the UK DRI centre at Imperial College London and the UK DRI Centre for Care Research and Technology at the University of Surrey.
Dr Karen Brakspear, Head of Neurosciences and Mental Health at MRC, said:
“Sleep disturbances are increasingly recognised as a risk factor for neurodegenerative disease such as Alzheimer’s disease.
By better understanding the brain’s transition into sleep, we can begin to unravel the complex links between sleep and dementia and potentially develop new strategies to promote healthy sleep.”
Study leader Dr Nir Grossman, Group Leader at the UK DRI at Imperial College London, said:
“Sleep is a fundamental part of our lives.
The process of falling asleep is the critical gateway to the vital physiological and cognitive benefits of sleep.
Yet, how our brain falls asleep has been one of the most enduring mysteries of neuroscience.
In this study, we presented a method that enables us, for the first time, to track in real-time how the brain transitions into sleep with unprecedented precision.
We discovered that falling asleep is a bifurcation, not a gradual process, with a clear tipping point that can be predicted in real time.
The ability to track how individual brains fall asleep has profound implications for our understanding of the sleep process.
It also serves as a vital health marker for changes in the brain, due to factors such as ageing or the development of brain diseases like dementia.
Importantly, it could facilitate the development of new treatments for people who struggle with falling asleep.”
Senior co-author Professor Derk-Jan Dijk, Group Leader at the UK DRI Centre for Care Research and Technology at Imperial College London and the University of Surrey, said:
“Our experiments demonstrate that by analysing brain waves through a ‘dynamical system’ lens, we can generate new insights into how sleep really works, going far beyond what standard sleep scoring reveals.
This study has the potential to transform how we clinically define the beginning of sleep.
It is commonly believed that falling asleep is a gradual, continuous process, but we have identified that it is in fact an abrupt transition, with a tipping point at which the brain moves from a waking state into sleep.
Understanding and harnessing this will enable us to better study the biology underpinning the process of falling asleep, and inform the development of new diagnostic tools and therapies.”
