It remains a mystery why we sleep, whether it’s for saving energy, clearing away the waste present at the brain, or that need for animals to be safe and sound from predators and external factors. But a pair of new studies just might enrich the discussion further.

Their key finding: people sleep to forget certain things learned every day. Learning, after all, entails growing neural connections, which enable neurons to send signals to each other efficiently. New memories are stored in these networks.

Shrinking Brain Synapses During Sleep

Back in 2003, University of Wisconsin-Madison biologists Drs. Giulio Tononi and Chiara Cirelli proposed that these synapses or neural connections grow so robustly at daytime that brain circuits become “noisy.” They argued that during sleep, the human brain cuts back on such connections to sift through the noise.

Over the years, they have discovered indirect proof supporting the so-called synaptic homeostasis hypothesis. Without this synaptic homeostasis, synapses could burn out akin to how an electrical outlet become overloaded with too many appliances plugged in.

"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.

Now, the scientists have direct evidence of their hypothesis after finding in a four-year experiment the shrinking of synapses in mouse models while they slept. Sleep then appears to be a method for the brain to keep learning new things and forming new memories.

 Further Evidence Of Synaptic Homeostasis

In the second study, a team from Johns Hopkins University explored the hypothesis by focusing on proteins in mouse brains. They fortified evidence that sleep recalibrates brain cells responsible for memory and learning, such that the animals can “solidify” lessons learned and use them once they wake up.

Creating a tiny window through which they could peek into mouse brains, the team targeted the hippocampus and cortex. They purified proteins from receiving synapses in sleeping and awake mice, finding a 20 percent decrease in receptor protein levels in sleeping mice. What this indicated was a general weakening of synapses in the sleeping bunch.

Lead author Dr. Graham Diering said their findings advance the idea that mouse — and presumably human — brain can only store so much before it demands recalibration.

“Without sleep and the recalibration that goes on during sleep, memories are in danger of being lost,” he said in a statement.

In the separate experiment of Tononi and Cirelli, the scientists found that pruning synapses in a lab dish did not strike every single neuron, with one-fifth of the synapses remaining unchanged. What this could mean: synapses encode the most stable and important memories that should not be altered or tampered.

Tononi dubbed it as forgetting “in a smart way,” and Cirelli believed this selective trait by the brain in pruning synaptic connections remains a mystery to explore.

Some researchers, however, remained cautious of the findings, saying they might not be definitive proof of the hypothesis at all. Sleep researcher Marcos G. Frank from Washington State University said, for instance, that it could be difficult to determine whether those brain changes at night result from sleep or by one’s biological clock.

The findings were discussed in the journal Science.

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