A Model of Sleep’s Role in the Brain

by Rabiya S. Tuma

September, 2006

Though findings agree that sleep is critical to our health and well-being, experiments do not show how sleep actually acts in the brain to make that the case. Giulio Tononi of the University of Wisconsin Medical School in Madison, however, thinks he knows what sleep does and why it is so necessary.

One key function of sleep is the reinforcement of memories and learning. During the course of a day our brains are constantly gaining information about what happens around us and storing that information in the form of altered synapses. With use, synapses become stronger and larger. But that process cannot continue indefinitely or our brains would run out of energy and space.

During slow-wave sleep, neurons fire for about three-quarters of a second and then are completely silent for about one-quarter of a second in a slow oscillatory pattern. The more a specific region of the brain was used during wakeful hours, the larger the slow waves are in that region during sleep. As sleep proceeds the size of the waves decreases.

Putting these observations together, Tononi hypothesizes that the slow-wave oscillations serve to scale down the synapses in the brain, but in a proportional manner. Thus the information remains stored but the synapses are returned to a more manageable size. He likens the process to a company that needs to periodically scale back in size to maintain its efficiency.

If his model is correct, then going without sleep would lead to decreasing brain efficiency. “Sleep is the price we pay for our brain’s plasticity when we are awake,” Tononi says.