The Interview: Sleep Expert Charles Czeisler
Can’t sleep? Boston’s top expert
explains why getting a good night’s rest means more than just a
comfortable pillow—it means changing the way we live.
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 machinery.
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.
That
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 the shock.
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 again.
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.
Marcos
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.
Markus
H. 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?”
Many
organs, not just the brain, seem to function differently during sleep,
Dr. Schmidt pointed out. The gut appears to make many new cells, for
example.
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 memories.
“You may actually work against yourself,” Dr. Tononi said.
In
the future, sleep medicines might precisely target the molecules
involved in sleep, ensuring that synapses get properly pruned.
“Once
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.
