Now new research from the University of Wisconsin School of Medicine and Public Health clarifies this phenomenon, supporting the idea that sleep plays a critical role in the brains ability to change in response to its environment. This ability, called plasticity, is at the heart of learning.
Reporting in the Jan. 20, 2008, online version of Nature Neuroscience, the UW-Madison scientists showed by several measures that synapses nerve cell connections central to brain plasticity were very strong when rodents had been awake and weak when they had been asleep.
The new findings reinforce the UW-Madison researchers highly-debated hypothesis about the role of sleep. They believe that people sleep so that their synapses can downsize and prepare for a new day and the next round of learning and synaptic strengthening.
The human brain expends up to 80 percent of its energy on synaptic activity, constantly adding and strengthening connections in response to all kinds of stimulation, explains study author Chiara Cirelli, associate professor of psychiatry.
Given that each of the millions of neurons in the human brain contains thousands of synapses, this energy expenditure is huge and cant be sustained.
We need an off-line period, when we are not exposed to the environment, to take synapses down, Cirelli say. We believe thats why humans and all living organisms sleep. Without sleep, the brain reaches a saturation point that taxes its energy budget, its store of supplies and its ability to learn further.
To test the theory, researchers conducted both molecular and electro-physiological studies in rats to evaluate synaptic potentiation, or strengthening, and depression, or weakening, following sleeping and waking times. In one set of experiments, they looked at brain slices to measu
|Contact: Dian Land|
University of Wisconsin-Madison