Why does the rustle of sheets wake us up on some nights, but we sleep through the sound of our alarm clocks going off on others? A new study implicates a type of brain activity known as alpha waves. With better monitoring of these waves, researchers might be able to develop therapies that could help all of us get a better night's sleep.
Our brain activity changes throughout the day. When we're awake, our neurons chatter in short, frequent bursts. Measured on an electroencephalogram (EEG), these "alpha waves" look a lot like earthquake squiggles on a seismograph. When we sleep, our neuron chatter slows down, resulting in less squiggly theta and delta waves. Scientists have shown that alpha waves help us respond to the sights and sounds of our environment. Yet they seem to disappear during sleep, even though we are still able to respond to environmental cues, such as smoke or a passing police siren.
So do alpha waves really disappear when we slumber? Sleep scientists Scott McKinney and Jeffrey Ellenbogen of Massachusetts General Hospital in Boston and colleagues used a sophisticated computer program to find out. Rather than eyeballing EEGs, as researchers had done in the past, the program teases apart the complicated patterns of brain activity. During sleep, the researchers found, alpha waves are still present—they're just drowned out by the theta and delta waves, like the din of a noisy restaurant drowns out individual conversations.
To find out what role these nighttime alpha waves play, McKinney and colleagues recruited 13 healthy subjects to spend several nights in the hospital's sleep lab. When the subjects' EEGs showed that they were sleeping deeply, the researchers played a variety of noises, from a jet engine to an ice machine. The noises started softly, at approximately 40 decibels—about as loud as a quiet home. The researchers gradually increased the noise until the person's EEG spiked, a sign that sleep had been disrupted.
"If you need a louder sound to wake them up, they're in a very deep phase of sleep," McKinney says. "If you only need a relatively quiet sound, they're in a more fragile or delicate phase of sleep." During these more fragile sleep phases, the hidden alpha waves were the strongest, the researchers report online today in PLoS ONE. So the strength of alpha waves was a good predictor of how easily a person could be roused from sleep.
"This is a very nice demonstration that there is a lot going on during sleep and a lot of short-term fluctuations," says psychiatrist Mathias Basner of the University of Pennsylvania Medical School in Philadelphia, who wasn't connected to the research. Sleep is usually divided into five different phases, and people were generally thought to move through these sleep phases in periods lasting 15 to 30 minutes. This study shows that the brain doesn't always sleep in these well-defined phases, Basner says.
McKinney hopes the findings will inform the development of more advanced sleep therapies. For example, drugs that will be administered only when a person is in danger of waking up, as determined by a small, portable EEG machine that might look like a headband. Today's sleep medications are very crude, McKinney says, and act kind of like a giant frying pan over the head, knocking you out for the whole night. "This [study] opens the door to administering a sleep drug using a real-time sleep monitor" similar to how doctors administer anesthetic, McKinney says.