Look at the clock. It’s 3 a.m. Why are you suddenly awake? You feel tired enough, your bed is cozy. You’re not even all that anxious.

Despite decades of lab tests in which human guinea pigs have napped, dozed, tossed and turned while researchers monitored their brains, sleep remains a fundamentally mysterious phenomenon. Questions abound as to why some of us seem to be so much better at it than others. But researchers have unearthed a new clue that could help us understand why some of us pop awake in the middle of the night—and how we might better treat that.

A new study points to the state of a particular brain wave that predicts how susceptible we are at any one moment to interruptions to our precious sleep. It’s called the “alpha wave,” and it’s a signal the brain typically emits when it is awake but relaxed. Scott McKinney, the lead author of the study, is the informatics manager at Massachusetts General Hospital sleep laboratory. He and his team wanted to look at these alpha waves in sleeping subjects.

“We know that there are times when sleep is more fragile and times when sleep is more stable,” McKinney says. “And so we wanted to be able to derive from an EEG some reliable indicator of the soundness of sleep and try to understand what that means.”

Alpha waves are often difficult to discern in a sleeping person; beneath the amalgam of brain signals, they become, as the research team puts it, “inconspicuous.” But using a technique common to electrical engineering called spectral analysis, the team was able to trace the fluctuations of their subjects’ alpha waves in far greater detail than with EEG alone, the technology that’s been used for almost a century to study the brain’s electrical output.

“The alpha rhythm during wakefulness is a very stark pattern that’s discernible with the naked eye,” McKinney says. Outside of waking states, though, “it becomes invisible, it’s buried within the other rhythms of sleep. We’re exhuming it, so to speak, with this mathematical technique.”

The alpha wave is "buried within the other rhythms of sleep. We’re exhuming it, so to speak, with this mathematical technique.”

Once they’d exhumed them, they wanted to determine what alpha waves might have to do with the nightly effort to tune out the waking world. So McKinney and his team recorded various noises--ringing phones, street traffic—the types of sounds that might typically wake a person up during the night. They then played these sounds next to their 13 sleep subjects, at increasingly louder volumes, until they successfully disrupted their subjects’ sleep, as measured by an EEG machine.

McKinney and colleagues observed that the more alpha activity their subjects had at the moment the noise was played, the more easily their sleep was interrupted.

“Moments of heightened alpha activity correspond to increased vulnerability of sounds," says McKinney. “And conversely, those reduced alpha expression correspond to soundness of sleep. “

This doesn’t mean that all insomniacs have overactive alpha waves. It seems that people who naturally produce more alpha activity are more prone to waking up. But the ability to fall asleep again could be something else entirely.

“Experiments,” McKinney says, “don't seem to bear out the notion that insomniacs are hypersensitive to noises in the night. It may be simply that they have a harder time returning to sleep after disturbance, and so are more apt to be irritated by disruption.”

What the findings could lead to, however, is a sea change in the way sleep disorders are treated. Currently, prescription sleep medications like Ambien and Lunesta keep the brain steadily sedated throughout the night. But the new research from Mass General’s sleep lab suggests that the brain only needs help staying asleep at certain times, and developing methods that deliver sleep assistance only during those times could be a valuable advance. That’s because, says McKinney, “When sleep is at its deepest and most resistant, maybe it’s best if you don’t interfere with it too much. We know there are various important things happening during sleep vital to cognition and memory, and some studies suggest that Ambien might interfere with those processes during deep sleep.”

Such innovation, alas, is still a twinkle in science’s eye. Theoretically, McKinney imagines, it would require the sleeper to wear a headband monitoring device to keep track of fluctuating alpha waves and trigger a pill to kick in when they hit a certain target threshold. This kind of approach to medicating sleep is for now “futuristic,” says Dr. Jeffrey Ellenbogen, the senior investigator on the study, so sheep on the night shift can breathe a sigh of relief.

Casey Schwartz is a graduate of Brown University and has a Masters Degree in psychodynamic neuroscience from University College London. She has previously written for The New York Sun and ABC News. Currently, she's working on a book about the brain world.