Sleep is the Price We Pay for Learning: New SHY Hypothesis

Why do we need sleep? Scientists have wondered just that for years. Now, researchers have found that their synaptic homeostasis hypothesis of sleep, or "SHY", challenges the theory that sleep strengthens brain connections. Instead, it's likely that 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.

In order to better understand the science of sleep, the researchers examined sleep and consciousness in animals ranging from fruit flies to humans. SHY takes into account evidence from molecular, electrophysiological and behavioral studies. The hypothesis itself suggests that in order to learn something, we need to sleep.

For example, assume that someone was learning how to play the piano. The circuits involved in learning this new skill would be greatly strengthened. The next day, though, the brain would need to pay attention to learning a new task. This means that the piano playing circuits would need to be damped down so that they wouldn't interfere with the new day's learning.

"Sleep is the price the brain must pay for learning and memory," said Giulio Tononi, one of the researchers, in a news release. "During wake, 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."

In fact, the reason why we don't forget how to play the piano after a night's rest is because active circuits are damped down less than those that weren't actively involved in learning. In fact, there's evidence that sleep enhances important features of memory, including acquisition, consolidation, gist extraction, integration and "smart forgetting," which allows the brain to rid itself of the inevitable accumulation of unimportant details. In addition, consolidation and integration of memories, as well as the restoration of the ability to learn, all come from the ability of sleep to decrease synaptic strength and enhance signal-to-noise ratios.

The findings are published in the journal Neuron.