Study shows synapses shrink during sleep to allow learning

Striking electron-microscope pictures from inside the brains of mice suggest what happens in our own brain every day: Our synapses – the junctions between nerve cells – grow strong and large during the stimulation of daytime, then shrink by nearly 20 percent while we sleep, creating room for more growth and learning the next day.

The four-year research project published in February in Science offers a direct visual proof of the “synaptic homeostasis hypothesis” (SHY) first proposed by Drs. Chiara Cirelli and Giulio Tononi of the Wisconsin Center for Sleep and Consciousness back in 2003.

This hypothesis holds that sleep is the price we pay for brains that are plastic and able to keep learning new things.

And it took four years of painstaking research to prove it. A large team of researchers sliced the brains of mice and then used a scanning electron microscope to photograph, reconstruct, and analyze two areas of cerebral cortex. They were able to reconstruct 6,920 synapses and measure their size.

The team deliberately did not know whether they were analyzing the brain cells of a well-rested mouse or one that had been awake. When they finally “broke the code” and correlated the measurements with the amount of sleep the mice had during the six to eight hours before the image was taken, they found that a few hours of sleep led on average to an 18 percent decrease in the size of the synapses. These changes occurred in both areas of the cerebral cortex and were proportional to the size of the synapses.

The study was big news, picked up by news outlets ranging from the New York Times to National Public Radio. It was bolstered by a companion Johns Hopkins University study that analyzed brain proteins to also confirm SHY’s prediction that the purpose of sleep is to scale back synapses.

For Cirelli, the study was a big gamble that paid off. But she’s not resting on her laurels. Her lab has already moved on to looking at new brain areas, and at the brain of young mice to understand the role sleep plays in brain development.