How is it that some people can sleep through sirens, blaring music or even an earthquake, while others will wake up if anyone so much as tiptoes into the room?
Scientists may now have an answer to this riddle, thanks to a study that cracks open one of the long-standing mysteries of getting a good night's sleep. Sound sleepers, they found, have specific patterns of activity in the brain that block out distracting signals during their slumber.
A gateway in the brain, called the thalamus, takes a role in the onward direction of most types of sensory information coming in to the brain. The researchers believe that brief bursts of electrical activity in this area, called "sleep spindles", somehow block external sounds form reaching any parts of the brain that might wake a person up. In short, light sleepers do not produce as many sleep spindles as heavy sleepers.
"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 in Boston. His results are published today in Current Biology.
"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," he said.
Light sleepers may benefit from boosting the number of spindles in their brains, though the researchers acknowledge they are still unclear how the boost could be achieved. Behavioural techniques, drugs, or an electronic device could provide a solution.
Ellenbogen's team recorded the brain-wave patterns of 12 volunteers who had to try to sleep in noisy laboratories over three nights. For the first night, they were undisturbed but, during the second and third nights, they were subjected to telephones ringing, people talking, and other mechanical sounds normally heard in hospitals.
"The effect of sleep spindles was so pronounced that we could see it even after just a single night," Ellenbogen said.
The scientists hope that understanding brain scans during sleep will enable researchers to develop tools to keep us asleep when we want to sleep and to wake us up only when it is time. "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," Ellenbogen said. "This finding gets us one important step closer to realising that goal."