Sound sleepers' brain waves block noise

Some people can sleep through anything. Phones ringing, doors slamming, cars rumbling can’t break through their slumber, while others wake at the slightest sound. Perhaps, suggests a new study in Current Biology, that's because some people’s brain wave patterns block noise from disrupting sleep.

Researchers from Massachusetts General Hospital tested 12 healthy volunteers in the hospital’s sleep lab for three nights. They looked at their brain rhythms with electroencephalograms, focusing on a particular pattern called spindles, which represent bursts of activity interspersed among slower periods.

Spindles are involved in how outside sensations -- including things we hear -- are transmitted from one part of the brain to another. During sleep, the researchers thought, spindles might block sounds, protecting people from shattered sleep.

For the first night, the study subjects slept hooked up to EEG electrodes in rooms equipped with sound machines to block out noise.

For the second and third nights, the subjects heard noise that had been recorded in a medical unit at Somerville Hospital, yielding the sounds of traffic, IV alarms, flushing toilets, conversations, snoring, an ice machine, and a helicopter, among other sonic challenges familiar to anyone who has spent a night in a hospital. Research staffers would turn on the sounds, gradually increasing the volume. Some subjects were aroused earlier than others but others didn’t wake up at all, no matter how loud the sound
got.

People whose EEGs showed more spindles while they were sleeping on the quiet night also had more spindles on the noisy nights than the other study subjects. The more spindles they produced, the less likely they were to wake up on the noisy nights.

“It is possible to predict which subjects are more disturbed by sounds in sleep by looking at brain activity and brain rhythms,” Dr. Thanh Dang-Vu said in an interview. “Here we show one important player is spindles: the more spindles, the more resistance to sounds.”

Scientists don't know why some people have more spindles than others, Dang-Vu said. Spindle production seems to be both individual and consistent, occurring amid quiet and noise. Further research might reveal ways to increase spindle rates through behavior, drugs, or devices. Some drugs that increase spindle rates have undesirable side effects, he said.