Scientists uncover how the brain falls asleep
Scientists have been able to pinpoint, for the first time, the exact moment the brain transitions into sleep, and precisely map the unfolding process in real time.
In the new study, the researchers demonstrated that the human brain falls asleep abruptly, rather than gradually, with a ‘tipping point’ marking the transition from wakefulness into sleep. They were then able to predict the momentary progression into sleep with unprecedented precision.
The findings could be used to develop new ways to diagnose and treat sleep disorders, such as insomnia, and as a marker of brain health in the context of ageing and neurodegenerative disease, and even to improve how we monitor anaesthesia during surgical procedures.
Led by the UK Dementia Research Institute (UK DRI) at Imperial College London and the UK DRI Centre for Care Research & Technology at the University of Surrey, and published in the journal Nature Neuroscience, the study analysed electroencephalogram (EEG) recordings taken from more than a thousand people who wore electrodes to record their brain activity during the night.
The researchers used a novel computational method that represented the changes in the brain as movement in a multi-dimensional space (called a feature space), and mapped out how each individual moved from bedtime toward sleep.
They found that no matter how long participants were in bed, the transition from wakefulness to sleep occurred abruptly in the final few minutes at a clear tipping point. Such a dynamic is known as a ‘bifurcation’, another example of which is the gradual bending of a stick until it eventually snaps. It also resembles the movement of a falling object, thereby supporting the subjective sensation of ‘falling asleep’.
In a different experiment, involving recording over multiple nights, they discovered that each participant had a unique location (in the multi-dimensional space) at which sleep began, which was consistent across multiple nights of sleep.
Armed with this discovery and the bifurcation model, the researchers showed that they could predict with 98% accuracy the exact progression (second-by-second) by which each participant fell asleep.
Previous definitions of sleep onset have relied on subjective, discrete annotations of brain activity from EEG readings, or indirect measures such as heart rate, breathing rate, and 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.
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.Study leader Dr Nir Grossman, Group Leader at the UK Dementia Research Institute at Imperial College London
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.
Sleep disturbances are increasingly recognised as a risk factor for neurodegenerative disease such as Alzheimer’s disease.Dr Karen Brakspear, Head of Neurosciences and Mental Health at the Medical Research Council
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.
This study was funded by the UK Dementia Research Institute, which is principally funded by the Medical Research Council.
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‘Falling asleep displays a predictable bifurcation dynamic’ by Li, J., Ilina, A., Peach, R., et al. is published in Nature Neuroscience. DOI: 10.1038/s41593-025-02091-1
The UK Dementia Research Institute (UK DRI) is a globally leading multidisciplinary research institute principally funded by the Medical Research Council (MRC). The UK DRI is dedicated to changing the outlook for people living with or at risk of neurodegenerative conditions, including Alzheimer’s, frontotemporal dementia, Huntington’s, Lewy body dementia, motor neuron disease, Parkinson’s, vascular dementia, and more. Its community of over 1000 researchers across nine world-class research centres are accelerating the discovery, development and delivery of interventions that will help diagnose, treat, and ultimately prevent neurodegeneration.
Established in 2017, the multi-million-pound Institute is nationally hosted across six leading UK universities: UCL, University of Cambridge, Cardiff University, University of Edinburgh, Imperial College London, and King’s College London. The institute is funded by the MRC, Alzheimer’s Society, Alzheimer’s Research UK, LifeArc, British Heart Foundation, NIHR and Parkinson’s UK. www.ukdri.ac.uk
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