Why do people and other animals sicken and die if they are deprived of sleep? What is it about sleep that makes it so essential?
A new study, published in Science, shows evidence that in fact humans sleep to forget some of the things they learn each day — maintaining the brain’s “plasticity,” its ability to change and adapt.
The investigation is a follow-up on the “synaptic homeostasis hypothesis” (SHY) posited by psychiatrists Drs. Chiara Cirelli and Giulio Tononi of the Wisconsin Center for Sleep and Consciousness. The research offers direct visual proof of SHY via electron-microscope pictures from inside the brains of mice. The visuals suggest what happens in our own brain every day.
The pictures showed that 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.
In the study, a large team of researchers sectioned the brains of mice, and then using a scanning electron microscope they photographed, reconstructed, and analyzed two areas of cerebral cortex. Investigators 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 has been bolstered by a companion Johns Hopkins University study that analyzed brain proteins, also confirming SHY’s prediction that the purpose of sleep is to scale back synapses.