"Sleep is important because it weakens the connections among brain cells to save energy, avoid cellular stress and maintain the ability of neurons to respond selectively to stimuli," said Giulio Tononi of the UW centre for sleep and consciousness.

Scientists developed a model called synaptic homeostasis hypothesis of sleep (SHY) challenging the theory that sleep strengthens brain connections.

During the waking hours, learning strengthens the synaptic connections throughout the brain, increasing the need for energy and saturating the brain with new information.

"Sleep allows the brain to reset, helping integrate, newly learned material with consolidated memories so the brain can begin anew the next day," said co-author Chiara Cirelli, professor of psychiatry.

'Synaptic homeostasis' refers to the brain's ability to maintain a balance in the strength of connections within its nerve cells, said the study published in journal Neuron.

Why would the brain need to reset?

Suppose someone spent the waking hours learning a new skill like riding a bike. The circuits involved in learning would be greatly strengthened, but the next day the brain will need to pay attention to learning a new task. Thus, those bike-riding circuits would need to be damped down so they don't interfere with the new day's learning, the study added.

"Sleep helps the brain renormalise synaptic strength based on a comprehensive sampling of its overall knowledge of the environment rather than being biased by the particular inputs of a particular waking day," said Tononi.


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