A Night Without Sleep Physically Reshapes Connections in the Brain
Sleep deprivation increased synaptic density markers across multiple human brain regions
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Why do we spend a third of our lives asleep?
A new study, led by researchers at Forschungszentrum Jülich, found that staying awake for 28 hours causes aphysical buildup of connectionsbetween brain cells. Their results provide the first direct evidence in living humans that a primary function of sleep is to reset overloaded neural pathways
The biological mystery of sleep
For generations, scientists have struggled to answer a fundamental question: why do we sleep?
While a lack of sleep degrades focus and mood, the underlying cellular purpose of this rest has remained an open mystery in neuroscience
To explain this, researchers proposed theSynaptic Homeostasis Hypothesis, known as SHY. This theory suggests that when we are awake and learning, our brains constantly strengthen the connections between neurons. However, these connections can crowd the brain’s limited physical capacity and keeping them strong requires energy. According to SHY, sleep solves this problem, systematically scaling down weaker and unimportant neural connections to restore balance.
Until recently, finding proof for this theory was difficult. Most supporting data came frominvasive animal models, such as counting brain cell spines in rodents or analyzing tissue samples. In humans, researchers had to rely on indirect methods like magnetic brain stimulation, and no one had directly captured these state-dependent changes in connection strength in a living human brain
In the new study, researchers usedpositron emission tomography (PET)imaging to quantify a molecular marker of connection density in humans after a period of acute sleep deprivation. They also aimed to determine if these structural brain changes directly correlated with physiological markers of sleep pressure
Sleep pressure
The biological urge to sleep that accumulates progressively every moment you are awake
Mapping the sleep-deprived brain
The randomized, test-retest study looked at a cohort of 40 healthy young adults, split into two groups: 20 individuals who maintained normal sleep, and 20 who faced acute sleep deprivation
The team used a tracer to monitor a protein found in brain connections, known as SV2A, which acts as an indicator of how crowded brain cell connections are. They scanned the sleep-deprived group at baseline and after 28 hours of wakefulness. They also measured attention with tasks, logged subjective sleepiness, and usedelectroencephalogramcaps to track slow-wave activity during a two-hour recovery nap.
Slow-wave activity
Deep, synchronized electrical waves that are generated by the brain during the heaviest, non-dreaming stages of sleep
Staying awake caused a widespread increase in the connection marker across six of eight brain regions analyzed. The marker rose by 5.6% in the hippocampus,which manages memory, 4.6% in the thalamus,the brain’s sensory relay station,and 3.2% in the parietal cortex,the brain’s primary control center for sensory integration and spatial awareness. The control group showed no variations between scans
“During sleep deprivation, the brain remains awake longer and continues to process stimuli and information. Our study shows that after approximately 28.5 hours of wakefulness, a marker for synaptic density increases in several brain regions. This suggests that sleep deprivation not only causes fatigue but is also accompanied by measurable changes in neural connections,” said the authors
Individual increases in the marker directly correlated with higher slow-wave activity during the recovery nap, suggestingthat sleep pressure is not just a feeling, but a direct response to a brain that has run out of space
No correlation was found between SV2A protein levels and attention lapses or subjective sleepiness
Connecting sleep, plasticity, and mood
The findings provide a structural biological explanation for why a lack of sleep leads to cognitive fatigue: When we stay awake, our brains become overloaded with strong connections that require a reset
“Given that synaptic deficits have been observed in major depression, and that both sleep deprivation and other fast-acting antidepressants (e.g., ketamine,electroconvulsive therapy) increase SV2A binding, our findings suggest a potential shared neurobiological mechanism linking synaptic plasticity, sleep, and mood regulation,” said the authors
However, the study has limitations. PET imaging cannot show whether the signal increase represents new connections, more proteins packed into existing terminals, or shifts between excitatory and inhibitory neurons. The sample size of 20 people per group is also small, which limits the ability to determine subgroup dynamics, such as sex-based variations in neural plasticity
Future work should look beyond a single night of sleep loss, allowing scientists to explore the impacts of chronic sleep restriction
“Chronic sleep restriction has been shown to have different adenosinergic dynamics compared with acute sleep deprivation. Longitudinal SV2A PET studies spanning recovery sleep and chronic sleep restriction protocols will be essential to determine whether these synaptic changes are fully reversible or carry cumulative neural costs,” said the authors
Reference:Griffa G, Palombo M, Yeffal A, et al.Learning engages transient and sustained cellular mechanisms in the human brain. Robertson EM, ed. PLoS Biol. 2026;24(6):e3003861. doi:10.1371/journal.pbio.3003861
This article is a rework of apress releaseissued by PLOS.Material has been edited for length and content.
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