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 brain 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 learning and memory,” says Dr. Giulio Tononi, of the UW Center for
Sleep and Consciousness. “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. “
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 fruit flies 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 renormalise
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 sleep during development, a time of intense
learning and massive remodeling of brain.
Source: sciencedaily.com
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