Sleeping shrinks the brain … and that’s a good thing

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The researchers found that sleep provides a time when the brain’s synapses — the connections among neurons — shrink back by nearly 20 percent. During this time, the synapses rest and prepare for the next day, when they will grow stronger while receiving new input — that is, learning new things, the researchers said. Without this reset, known as “synaptic homeostasis,” synapses could become overloaded and burned out, like an electrical outlet with too many appliances plugged in to it, the scientists said.
“Sleep is the perfect time to allow the synaptic renormalization to occur … because when we are awake, we are ‘slaves’ of the here and now, always attending some stimuli and learning something,” said study co-author Dr. Chiara Cirelli of the University of Wisconsin-Madison Center for Sleep and Consciousness.
“During sleep, we are much less preoccupied by the external world … and the brain can sample [or assess] all our synapses, and renormalize them in a smart way,” Cirelli told Live Science. Cirelli and her colleague, Dr. Giulio Tononi, also of the University of Wisconsin-Madison, introduced this synaptic homeostasis hypothesis (SHY) in 2003.
Now, Cirelli and Tononi have direct visual evidence of SHY after observing the shrinking of synapses in mice while the animals slept, an intricate experiment spanning four years. The researchers described their findings today (Feb. 2) in the journal Science.
Sleep is the price people pay for brains that are able to keep learning new things, the researchers said. Russell Foster, who directs the Sleep and Circadian Neuroscience Institute at the University of Oxford in the United Kingdom, who was not associated with the study, called it a “very nice, clear piece of work.” The findings support the notion that sleep is necessary for the consolidation of memories and thus learning, Foster said.
For millennia, humans have probed the nature and purpose of sleep. Aristotle suggested that sleep was restorative, a time to replace or rebuild all that was burned up throughout the body during the day. Modern science supports this idea, with researchers identifying sets of genes associated with restoration and metabolic pathways that turn on only during sleep. Cirelli and Tononi focused on sleep’s effect on the brain. In a paper published in 2003, they hypothesized about sleep’s role in the growth of synapses, which serve as avenues to ferry information among neurons.