What happens to your brain after more than 28 hours without sleep

Poor sleep often shows up in small details: having to reread the same sentence several times, forgetting why you walked into a room or taking longer than usual to answer a simple question. Attention becomes less stable, memory feels less reliable and every task requires a little more effort. However, reducing all of this to tiredness leaves out an important part of what is happening. Sleep deprivation does not only change how we feel, but also the way the brain manages its connections and preserves its ability to keep learning. A recent study has found that staying awake for more than 28 hours leaves measurable changes in that system.

The study, published in PLOS Biology, involved 40 healthy adults between the ages of 20 and 45. Half slept normally, while the other half remained awake for around 28 and a half hours. The researchers then analysed their brains using positron emission tomography and magnetic resonance imaging. They were looking for changes in a protein called SV2A, which is found in the structures that allow neurons to release neurotransmitters and communicate with one another. Rather than simply asking whether the participants felt tired, the team wanted to determine whether sleep deprivation physically altered a marker associated with synapses, the points of communication between neurons.

The results showed an increase in this signal in six of the eight brain regions analysed. The changes were particularly visible in the hippocampus, thalamus and parietal cortex, areas associated with memory, attention and information processing. It may seem strange that lack of sleep increases a marker linked to neural connections. After all, we tend to associate greater brain activity with something positive. But more activity does not necessarily mean better functioning. Under certain conditions, it may indicate that the system has not had the opportunity to reset.

Throughout the day, every conversation, decision, stimulus or learning experience modifies communication between neurons to some extent. Some connections are strengthened because they contain useful information, while others are activated more temporarily. If this process continued to build up without any form of regulation, neural networks would lose some of their ability to distinguish which signals matter. The brain would continue to receive information, but separating it from background noise would become more difficult. One of the functions of sleep may be precisely to reduce some of this accumulated intensity and preserve the most relevant changes.

This explanation forms part of the synaptic homeostasis hypothesis. In simple terms, it proposes that wakefulness progressively increases the strength of many connections, while sleep helps readjust them. This does not mean that the brain erases what it has learned during the day or that all synapses weaken equally. The process is more like regulating a system that has been working for many hours: lowering what has become excessively amplified, preserving useful signals and preventing the entire network from continuing to operate at maximum intensity. Without that reset, the brain can remain active, but it has less capacity to incorporate new information accurately.

That is why, when someone stays awake for an entire night, the problem is not simply that they think more slowly: it becomes harder to sustain attention, detect errors or react to unexpected stimuli. In the study, sleep-deprived participants showed greater sleepiness and poorer performance in a psychomotor vigilance test used to measure lapses in attention and response speed. In everyday life, this loss of precision may appear while driving, studying, working with data or carrying out a repetitive task. The brain does not stop functioning, but it alternates between periods of acceptable performance and brief lapses in which the response comes too late or does not come at all.

Memory depends on the same balance. Learning something new requires the brain to be able to modify certain connections without the entire system already being overloaded. Sleep helps organise some of the information acquired during the day, strengthen certain memories and reduce interference. When rest is missing, two difficulties combine: the brain loses an important phase for processing what has been learned and also faces the following day with less capacity to register new information. This helps explain why studying through the night does not guarantee better recall, even if more hours are spent on the subject.

Deep sleep appears to play a particularly important role in this recovery. After the sleepless night, the study participants were allowed to take a two-hour nap. Those who showed the greatest increases in the SV2A marker also displayed more intense slow-wave activity, which is characteristic of the deeper stages of sleep. These waves tend to increase when sleep pressure is high and the brain needs to recover. The relationship observed suggests that, after many hours awake, the body intensifies the mechanisms associated with deep rest.

However, a nap does not prove that all of the changes disappear immediately. The researchers did not perform another scan after a full night of recovery sleep, so they could not determine how long the brain signal took to return to its initial state. The study also does not show that occasionally sleeping one hour less produces exactly the same effect as staying awake all night. It examines a specific situation of acute sleep deprivation in healthy adults. Even so, it raises an important question: what may happen when the brain repeatedly goes through days without enough time to reset.

The study does not allow these findings to be applied to every night of poor sleep, but it does show that remaining awake for more than 28 hours leaves a measurable mark on the brain. What we experience as lapses, slowness or lack of clarity is not simply a feeling of tiredness. It may be the everyday expression of a system that is still functioning without having completed its reset and that has less capacity to filter information, learn and respond accurately.