Two sleep scientists from the University of Wisconsin School of Medicine and Public Health have introduced a hypothesis that challenges the theory that sleep strengthens brain connections.
Instead, the synaptic homeostasis hypothesis of sleep — SHY — 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 learning and memory,” says Dr. Giulio Tononi, of the university’s Center for Sleep and Consciousness.
“When awake, 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. ”
Why would the brain need to reset?
Suppose you spent your 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. That means those bike-riding circuits would need to be damped down so they don’t interfere with the new day’s learning, Tononi says.
“Sleep helps the brain renormalize 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,” he explains.
The reason we don’t 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, he adds.
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, he continues.
However, one common belief is that sleep helps memory by further strengthening the neural circuits during learning while awake.
But Tononi and his co-author Dr. Chiara 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.
Tononi and Cirelli’s laboratory studies sleep and consciousness in animals ranging from fruit flies to humans. They note that 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, the researchers explain.
In a review article in the journal Neuron, the researchers say that while there is evidence for the SHY hypothesis, it also points to open issues. One question is whether the brain could achieve synaptic homeostasis while awake, by having only some circuits engaged and the rest off-line and resetting themselves.
Other areas for future research include the specific function of REM sleep — when most dreaming occurs — and the possibly crucial role of sleep during development, a time of intense learning and massive remodeling of brain
Source: University of Wisconsin-Madison