A new person every time you wake up? A new study suggests that your brain resets itself during sleep. The findings published in the journal Science.
Our brains are reset during sleep, the latter being the price we pay to have brains that are able to accommodate for the assimilation of new things—a theory called the “synaptic homeostasis hypothesis” (SHY). The way this happens has been revealed in electron microscope pictures.
When researchers from University of Wisconsin-Madison looked into the brains of mice, they found that during sleep, the brain would undergo changes that allowed for a greater growth, and more efficient learning upon waking up. This ‘resetting’ is because synapses—the gaps lying between neurones (nerve cells)—are stimulated during daytime, becoming large and strong, to eventually shrink by around 20% during sleep, to boost growth and learning.
Synapses are constantly activated when one is awake. This repeated pattern makes them stronger and stronger as well as bigger and bigger. However, according to SHY, this growth has to be kept in check so that the structures in question are not overwhelmed, a state that would potentially lead to the disruption of neural signalling, and memory deletion. To avoid this, humans sleep. During this period, the opposite of what happens during daytime occurs: synapses shrink as they weaken. The researchers wanted to test this hypothesis, and they endeavoured to measure the size of synapses during sleep and wakefulness.
The methodology included the use of serial scanning 3-D electron microscopy, a technique that offered a high spatial resolution, which allowed the team of scientists to measure the tiny structure, the synapse. Thus, 6,920 synapses from two areas of the mouse brain were photographed, reconstructed, and measured.
As the researchers compared the measurements with the amount of time spent sleeping prior to the photographing, they found that few hours of sleep were correlated with an 18% decreased synapse size. This reduction in size—seen in most of the synapses—was proportional to the size of the synapse. The largest synapses were, however, left untouched: they did not undergo the shrinking. This might be because of their greater importance as they might be linked with the most stable traces of memory.
“This shows, in unequivocal ultrastructural terms, that the balance of synaptic size and strength is upset by wake and restored by sleep,” says author Chiara Cirelli. “It is remarkable that the vast majority of synapses in the cortex undergo such a large change in size over just a few hours of wake and sleep.”
“Extrapolating from mice to humans, our findings mean that every night trillions of synapses in our cortex could get slimmer by nearly 20 percent,” remarks co-author Giulio Tononi.