A type of brain activity known as an alpha wave emanates from the back of the head when a person is awake but relaxing with eyes closed. Scientists used to think that the wave was subdued and disappeared as a person fell deeper and deeper into sleep.
But the alpha wave doesn’t disappear; it just goes undercover during sleep, researchers report online March 3 in PLoS One. The covert alpha wave may help determine how deeply people sleep and how much noise is needed to rouse a sleeper.
The finding “stresses that sleep is really a dynamic process,” says Mathias Basner, a sleep researcher at the University of Pennsylvania in Philadelphia who was not involved in the study. The study shows that sleep doesn’t happen just in discrete blocks, as most charts of sleep stages would indicate. Instead, brain activity changes from moment to moment during sleep.
“It may suggest that something is going on in the central nervous system that we don’t know about and should maybe pay more attention to,” Basner says.
Scientists hadn’t ignored alpha waves on purpose, says study coauthor Scott McKinney, a sleep scientist at Massachusetts General Hospital and Harvard University. Researchers typically measure brain activity during sleep with electroencephalographs, or EEGs, devices that use electrodes on the scalp to detect electrical activity in the brain.
The squiggly lines recorded by the EEG can be hard to interpret with the naked eye, so McKinney and his colleagues used computer programs to break the EEG signals from 13 volunteers down into discrete waves. The analysis revealed that alpha waves never truly go away; they just get drowned out by more vigorous signals the way spreading ripples from a small rock dropped in a pond are swamped by waves from a passing speedboat.
Alpha wave activity decreases as people enter ever-deeper levels of sleep and increases as people cycle back into more shallow sleep stages. In study participants, the ups and downs of alpha wave activity were closely associated with how easily a person could be awoken by traffic noises, loud talking or other sounds that might be encountered in hospital or at home in a city. When alpha wave activity spiked just before a noise was played, volunteers woke up more easily than when alpha wave activity was low, the researchers found.
Alpha wave activity may be the brain’s way of keeping people aware of their surroundings during sleep, speculates Phyllis Zee, director of the Sleep Disorders Program at Northwestern University in Chicago. Such awareness enables people to wake quickly in case of danger, but too much alpha activity might also have a downside if it prevents a good night of sleep.
People with insomnia commonly complain that they are very light sleepers and are always aware of their surroundings, Zee says. Although many insomniacs get a full night of sleep, they report that their sleep is not restful. But laboratory tests often don’t show any abnormalities.
“The classical way we’re scoring sleep may not give a good handle on what a patient really experiences,” she says. “This new way of analyzing depth of sleep may be used to get a better understanding of a patient’s complaint.”