Spindles foster sound slumber

Distinctive brain signals sustain sleep in noisy environments

Sound sleepers make more waves. Certain types of brain waves, that is.

Brain waves known as sleep spindles help keep sleepers slumbering even when confronted with airplane noise, traffic sounds, ringing telephones and loud conversations. A new study shows that people who can sleep through all of that produce more spindles even on quiet nights than light sleepers do.

The study, published in the Aug. 10 Current Biology, suggests that boosting spindles might help poor sleepers get better shut-eye.

“The goal of this research agenda is to make the sleeping environment really a utopia,” says Jeffrey Ellenbogen, a neurologist and sleep specialist at Harvard Medical School and Massachusetts General Hospital in Boston. But in order to achieve that goal, the researchers needed to know what happens during a night of good sleep.

A dozen healthy volunteers came to the sleep lab for three consecutive nights. On the first night, all was quiet and researchers measured electrical activity in the volunteers’ brains with an electroencephalogram, or EEG.

The team was especially interested in how many spindles each person had throughout the night, because spindles are known to be part of the brain’s response to shutting out external stimuli. The electrical signatures — called spindles because the squiggly patterns they produce on an EEG trace resemble a spindle of yarn — originate in a part of the brain called the thalamus, which serves as a sort of toll booth for all types of sensory information except smell.

Spindles may be a sign that the thalamus is lowering a barrier to prevent disruptive sounds, touches or internal signals such as the oxygen content of the blood from disrupting sleep. Some researchers think that it is necessary for the brain to shut out the outside world during sleep in order to process memories properly (SN: 10/24/09, p. 16).

But even on quiet nights, some people have far more spindles than others do. The more spindles a person had on a quiet night, the harder it was for researchers to blast the person awake with airplane noises and hospital sounds the next two nights. People with fewer spindles woke up more easily.

“It’s a very clever study,” says Mathias Basner, a sleep researcher at the University of Pennsylvania in Philadelphia. Researchers have previously associated spindles with blocking out sound, but this study suggests that spindles are an inborn characteristic of each person. Basner says it is too soon to tell whether the number of spindles a person’s brain produces each night is a trait that is set in stone, or if the brain can train itself to deal with noisy environments.

People often claim that train noises or other loud sounds don’t disturb their slumber once they’ve gotten used to the din. But Ellenbogen is skeptical that people can train their brains to handle exposure to loud noises well when sleeping.  

Volunteers barely registered disturbances to their sleep in the new study. Before people left the lab in the morning, the researchers asked about the quality of sleep from the night before. Volunteers recalled being awakened a couple of times during the night by loud sounds, when in fact researchers had played 60 sounds and woken the people up 30 to 40 times.

Drugs or other treatments might one day be developed to boost the number of spindles a person produces each night, Ellenbogen says, but researchers would need to be sure that any spindles produced by such therapies actually do the same thing as naturally occurring spindles. “Synthetic sleep is not something that is attractive to me,” he says. “What I want is to enhance natural sleep.”

But while boosting spindles might help some people get a better night’s sleep, raising the amount of noise a person can tolerate during sleep might not be a good idea for everyone, Basner says. For instance, studies have shown that among people who stop breathing during sleep — a condition known as apnea — those who startle awake more easily fare better than those who stay asleep longer. That is probably because people who fail to rouse are depriving themselves of oxygen longer.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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