Cafeneaua PEROMANESTE dela voi pentru voi: The Purpose of Sleep? To Forget, Scientists Say:
Over the years, scientists have come up with a lot of ideas about why we sleep.
have argued that it’s a way to save energy. Others have suggested that
slumber provides an opportunity to clear away the brain’s cellular
waste. Still others have proposed that sleep simply forces animals to
lie still, letting them hide from predators.
pair of papers published on Thursday in the journal Science offer
evidence for another notion: We sleep to forget some of the things we
learn each day.
In order to learn, we have
to grow connections, or synapses, between the neurons in our brains.
These connections enable neurons to send signals to one another quickly
and efficiently. We store new memories in these networks.
In 2003, Giulio Tononi and Chiara Cirelli, biologists at the University of Wisconsin-Madison, proposed that synapses grew
so exuberantly during the day that our brain circuits got “noisy.” When
we sleep, the scientists argued, our brains pare back the connections
to lift the signal over the noise.
years since, Dr. Tononi and Dr. Cirelli, along with other researchers,
have found a great deal of indirect evidence to support the so-called
synaptic homeostasis hypothesis.
out, for example, that neurons can prune their synapses — at least in a
dish. In laboratory experiments on clumps of neurons, scientists can
give them a drug that spurs them to grow extra synapses. Afterward, the
neurons pare back some of the growth.
evidence comes from the electric waves released by the brain. During
deep sleep, the waves slow down. Dr. Tononi and Dr. Cirelli have argued
that shrinking synapses produce this change.
years ago, Dr. Tononi and Dr. Cirelli got a chance to test their theory
by looking at the synapses themselves. They acquired a kind of deli
slicer for brain tissue, which they used to shave ultrathin sheets from a
Luisa de Vivo, an
assistant scientist working in their lab, led a painstaking survey of
tissue taken from mice, some awake and others asleep. She and her
colleagues determined the size and shape of 6,920 synapses in total.
synapses in the brains of sleeping mice, they found, were 18 percent
smaller than in awake ones. “That there’s such a big change over all is
surprising,” Dr. Tononi said.
The second study was
led by Graham H. Diering, a postdoctoral researcher at Johns Hopkins
University. Dr. Diering and his colleagues set out to explore the
synaptic homeostasis hypothesis by studying the proteins in mouse
brains. “I’m really coming at it from this nuts-and-bolts place,” Dr.
In one experiment, Dr. Diering
and his colleagues created a tiny window through which they could peer
into mouse brains. Then he and his colleagues added a chemical that lit
up a surface protein on brain synapses.
through the window, they found that the number of surface proteins
dropped during sleep. That decline is what you would expect if the
synapses were shrinking.
Dr. Diering and his
colleagues then searched for the molecular trigger for this change.
They found that hundreds of proteins increase or decrease inside of
synapses during the night. But one protein in particular, called
Homer1A, stood out.
In earlier experiments
on neurons in a dish, Homer1A proved to be important for paring back
synapses. Dr. Diering wondered if it was important in sleep, too.
find out, he and his colleagues studied mice genetically engineered so
that they couldn’t make Homer1A proteins. These mice slept like ordinary
mice, but their synapses didn’t change their proteins like the ones in
Dr. Diering’s research
suggests that sleepiness triggers neurons to make Homer1A and ship it
into their synapses. When sleep arrives, Homer1A turns on the pruning
To see how this pruning machinery
affects learning, the scientists gave regular mice a memory test. They
put the animals in a room where they got a mild electric shock if they
walked over one section of the floor.
night, the scientists injected a chemical into the brains of some of the
mice. The chemical had been shown to block neurons in dishes from
pruning their synapses.
The next day, the
scientists put all the mice back in the chamber they had been in before.
Both groups of mice spent much of the time frozen, fearfully recalling
But when the researchers put the
mice in a different chamber, they saw a big difference. The ordinary
mice sniffed around curiously. The mice that had been prevented from
pruning their brain synapses during sleep, on the other hand, froze once
Dr. Diering thinks that the injected
mice couldn’t narrow their memories down to the particular chamber
where they had gotten the shock. Without nighttime pruning, their
memories ended up fuzzy.
In their own
experiment, Dr. Tononi and his colleagues found that the pruning didn’t
strike every neuron. A fifth of the synapses were unchanged. It’s
possible that these synapses encode well-established memories that
shouldn’t be tampered with.
“You can forget in a smart way,” Dr. Tononi said.
Other researchers cautioned that the new findings weren’t definitive proof of the synaptic homeostasis hypothesis.
G. Frank, a sleep researcher at Washington State University in Spokane,
said that it could be hard to tell whether changes to the brain at
night were caused by sleep or by the biological clock. “It’s a general
problem in the field,” he said.
Schmidt, of the Ohio Sleep Medicine Institute, said that while the brain
might prune synapses during sleep, he questioned whether this was the
main explanation for why sleep exists.
“The work is great,” he said of the new studies, “but the question is, is this a function of sleep or is it the function?”
organs, not just the brain, seem to function differently during sleep,
Dr. Schmidt pointed out. The gut appears to make many new cells, for
Dr. Tononi said that the new
findings should prompt a look at what current sleeping drugs do in the
brain. While they may be good at making people sleepy, it’s also
possible that they may interfere with the pruning required for forming
“You may actually work against yourself,” Dr. Tononi said.
the future, sleep medicines might precisely target the molecules
involved in sleep, ensuring that synapses get properly pruned.
you know a little bit of what happens at the ground-truth level, you
can get a better idea of what to do for therapy,” Dr. Tononi said.