Ah, to sleep, perchance … to shrink your neural connections? That's the conclusion of new research that examined subtle changes in the brain during sleep.
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. [10 Things You Didn't Know About the Brain]
"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. Synapses are constantly strengthening, or widening, during the day to accommodate the flow of traffic as the brain soaks up new experiences. But that strengthening cannot go on indefinitely, or else the synapses will become saturated—think "information overload."
The researchers suggested in their earlier paper that synapses get pruned back during sleep. This pruning doesn't necessarily cause the body to need sleep; rather, the body is taking advantage of the decreased brain traffic that occurs while an individual sleeps. [7 Mind-Bending Facts About Dreams]
To find evidence for this, the researchers used a new form of electron microscopy that can discern the miniscule changes in the shrinking and subsequent expansion of these microscopic synapses at the nanometer level in mice brains. They found that a few hours of sleep led to an 18 percent decrease in the size of the synapses on average.
Cirelli said that one interesting finding was that this pruning occurred in about 80 percent of the synapses but spared the largest ones. These larger synapses may be associated with the most stable and important memories, connections the brain does not want to lose, the researchers speculated. Yet, the way in which the brain decides what synaptic connections to prune is another mystery to explore, Cirelli said.
"It is critical to have pruning back at night, so that the huge amount of information encoded by temporary synapses during the day won't overwhelm the brain," said Foster. "Pruning ensures that only the most important information is retained."
Foster said he can envision follow-on experiments based upon the Cirelli-Tononi work that would use mouse models to explore the connections among circadian rhythms (the body's "internal clock"), sleep, synapse pruning and psychiatric disorders. Some of the key features of these disorders seem to be a disruption in neural circuitry, sleep disruption, and impaired cognition and memory, said Foster, who is also a co-author of the upcoming book "Circadian Rhythms: A Very Short Introduction," (Oxford University Press, 2017).
Foster added that resetting synapses may be a core feature of sleep, particularly for humans, with their advanced cognitive abilities compared to other animals. However, pruning is likely to be just one of many essential functions that takes place during the sleep phase, a period during which the body takes advantage of physical inactivity to perform a range of essential housekeeping activities, he said.
So Aristotle wasn't too far off.
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