SHY hypothesis explains that sleep is the price we pay for learning

January 9, 2014

Why do animals ranging from fruit flies to humans all need to sleep? After all, sleep disconnects them from their environment, puts them at risk and keeps them from seeking food or mates for large parts of the day.

Two leading sleep scientists from the University of Wisconsin School of Medicine and Public Health say that their synaptic homeostasis hypothesis of sleep or "SHY" challenges the theory that sleep strengthens connections. The SHY hypothesis, which takes into account years of evidence from human and animal studies, says 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.

"Sleep is the price the brain must pay for and memory," says Dr. Giulio Tononi, of the UW Center for Sleep and Consciousness. "During wake, learning strengthens the 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. "

Tononi and his co-author Dr. Chiara Cirelli, both professors of psychiatry, explain their hypothesis in a review article in today's issue of the journal Neuron. Their laboratory studies sleep and consciousness in animals ranging from to humans; SHY takes into account evidence from molecular, electrophysiological and behavioral studies, as well as from computer simulations. "Synaptic homeostasis" refers to the brain's ability to maintain a balance in the strength of connections within its nerve cells.

Why would the brain need to reset? Suppose someone spent the waking hours learning a new skill, such as 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.

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

The reason we don't also forget how to ride a bike after a night's sleep is because those active circuits are damped down less than those that weren't actively involved in learning. Indeed, there is 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. However, one common belief is that sleep helps memory by further strengthening the neural circuits during learning while awake. But Tononi and Cirelli believe that 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.

While the review finds testable evidence for the SHY hypothesis, it also points to open issues. One question is whether the brain could achieve synaptic homeostasis during wake, by having only some circuits engaged, and the rest off-line and thus resetting themselves. Other areas for future research include the specific function of REM sleep (when most dreaming occurs) and the possibly crucial role of during development, a time of intense learning and massive remodeling of brain.

Explore further: Even in fruit flies, enriched learning drives need for sleep

Related Stories

Even in fruit flies, enriched learning drives need for sleep

June 23, 2011
Just like human teenagers, fruit flies that spend a day buzzing around the "fly mall" with their companions need more sleep. That's because the environment makes their brain circuits grow dense new synapses and they need ...

Connections in the brains of young children strengthen during sleep, study finds

November 20, 2013
While young children sleep, connections between the left and the right hemispheres of their brain strengthen, which may help brain functions mature, according to a new study by the University of Colorado Boulder.

Sleep to protect your brain

December 31, 2013
A new study from Uppsala University, Sweden, shows that one night of sleep deprivation increases morning blood concentrations of NSE and S-100B in healthy young men. These molecules are typically found in the brain. Thus, ...

Neuroscientist not napping on idea of explaining sleep

November 8, 2013
Don't nod off on this one. A Western neuroscientist is exploring the possibility sleep isn't so much about rest from a busy day as it is about memory consolidation – or, more simply, the process needed to form lasting memories.

If you don't snooze, do you lose? Wake-sleep patterns affect brain synapses

October 9, 2011
An ongoing lack of sleep during adolescence could lead to more than dragging, foggy teens, a University of Wisconsin-Madison study suggests.

Recommended for you

Team traces masculinization in mice to estrogen receptor in inhibitory neurons

July 20, 2017
Researchers at Cold Spring Harbor Laboratory (CSHL) have opened a black box in the brain whose contents explain one of the remarkable yet mysterious facts of life.

New study reveals contrasts in how groups of neurons function during decision making

July 19, 2017
By training mice to perform a sound identification task in a virtual reality maze, researchers at Harvard Medical School and the Istituto Italiano di Tecnologia (IIT) have identified striking contrasts in how groups of neurons ...

Memory takes time, researchers conclude

July 19, 2017
How short-term memories become long-term ones has frequently been explored by researchers. While a definitive answer remains elusive, New York University scientists Thomas Carew and Nikolay Kukushkin conclude that this transformation ...

Researchers identify new target for chronic pain

July 19, 2017
Proteins must be in the right place at the right time in the cell to function correctly. This is even more critical in a neuron than in other cells because of its complex tree-like structure and its function. Researchers ...

Brains are more plastic than we thought

July 19, 2017
Practice might not always make perfect, but it's essential for learning a sport or a musical instrument. It's also the basis of brain training, an approach that holds potential as a non-invasive therapy to overcome disabilities ...

Healthy heart in 20s, better brain in 40s?

July 19, 2017
Folks with heart-healthy habits in their 20s tend to have larger, healthier brains in their 40s—brains that may be better prepared to withstand the ravages of aging, a new study reports.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.