During sleep, brain waves become slow and organized and the thalamus, a way station for all types of sensory information except smell, spontaneously engages with the cortex. This interaction can produce transient fluctuations of the brain's electric field visible on the EEG as rhythmic spindles - brief bursts of faster-frequency waves.
"We found that by measuring brain waves during sleep, we could learn a lot about how well a person's brain can block the negative effects of sounds; the more sleep spindles your brain produces, the more likely you'll stay asleep, even when confronted with noise," said Jeffrey Ellenbogen of Harvard Medical School. "The thalamus is likely preventing sensory information from getting to areas of the brain that perceive and react to sound. And our data provide evidence that the sleep spindle is a marker of this blockade. More spindles means more stable sleep, even when confronted with noise."
Ellenbogen said they were surprised at the magnitude of the sleep spindle effect. They observed brain patterns of study participants as they slept in the lab for three nights. The first night was quiet and the second and third nights were noisy, as the researchers introduced a variety of sounds—a telephone ringing, people talking, hospital-based mechanical sounds, and so on. "The effect of sleep spindles was so pronounced that we could see it even after just a single night," he said.
The researchers say they hope to devise ways to enhance sleep spindles via behavioral techniques, drugs, or devices, but it's not yet clear how to do that.
Ellenbogen said such advances would be particularly welcome today, as "our sleeping environments have gotten increasingly complex and problematic, with all the beeps and boops of our 24/7 modern, crowded lives. And there are particular challenges in a hospital setting where some of the sounds are necessary (e.g., heart monitors need to send an alarm if there's a problem). Our goal is to find brain-based solutions that integrate a sleeping person into their modern environment, such that sleep is maintained even in the face of noises. This finding gets us one important step closer to realizing that goal."
Ellenbogen ultimately envisions a future in which we'll have access to multiple strategies, based on sound sleep science and technologies, to help keep us asleep when we want to sleep and awaken us when it's time to get up. "In the meantime," he said, "it still doesn't hurt to put up a sign that says 'Shhh!'"
Another piece of advice for those who really must go to sleep with the radio or TV on: use a timer. The researchers' evidence shows that such noises do disrupt sleep, whether the sleeping person realizes it or not.
Citation: Thien Thanh Dang-Vu, Scott M. McKinney, Orfeu M. Buxton, Jo M. Solet, and Jeffrey M. Ellenbogen, 'Spontaneous brain rhythms predict sleep stability in the face of noise', Current Biology, Volume 20, Issue 15, R626-R627, 10 August 2010 doi:10.1016/j.cub.2010.06.032