Our eyes swivel restlessly in their sockets during rapid eye movement (REM) sleep, an aptly named period of intense dreaming that makes up 20 to 25 percent of total sleep time. Whether this fidgeting is random or serves a function has never been clear, but a new study suggests that our eyes shift their gaze to fixate on the imagined people, places and actions in our sleep dreamscape. In other words, the movements of dreaming eyes mimic those of waking eyes.

For more than 50 years neuroscientists have debated the reasons for REM during sleep, proposing all kinds of ideas: the eyes roll around to lubricate the inside of the eyelids; jiggling eyes warm the brain; eyes twitch randomly in response to stimulation from the brain stem. According to a study in the June issue of Brain, the most likely explanation is the "scanning hypothesis," which says that throughout REM sleep our eyes orient their gaze to scan the imagery of our dreams—just as eyes change their gaze in response to our environment when we're awake and moving around.

But how do researchers know where someone is looking while they dream? And how do they test whether that has anything to do with what someone is dreaming about?

In the study neuroscientists at Pitié-Salpêtrière Hospital in Paris turned to a unique group of subjects: individuals with REM sleep behavior disorder (RBD). During REM sleep, the limb muscles of most people enter a state of temporary paralysis that prevents any flailing about. People with RBD do not go into sleep paralysis and physically act out their dreams—often with dramatic and violent actions: They kick, scream, grab, reach, climb and jump, both in their dreams and in reality, allowing researchers to observe what normally remains inside a dreamer's head.

"It's a direct window on people's dreams," says Isabelle Arnulf, a neurologist who specializes in sleep and a co-author of the Brain study. "It's kind of like having movie subtitles."

Arnulf and her colleagues used electrodes to track the eye movements of 56 people with RBD and 17 normal sleepers, simultaneously videotaping their nighttime behaviors. The team first compared the rapid eye movement of the two groups, to make sure that those with RBD did not have abnormal eye movements when dreaming, and found that both groups shifted their dreaming gazes with highly similar patterns and frequencies.

Then the researchers analyzed the nighttime footage of the RBD subjects frame by frame to see if their actions and gazes matched up. For example, would a subject who dreamt of playing in the outfield actually look in the direction of an imaginary incoming baseball? For 90 percent of the time an RBD sleeper's gaze synchronized with mimed dream actions.

A subject who dreamt of kissing someone to her left also looked to her left. Another participant who dreamt of climbing a ladder shifted his gaze up and down repeatedly to check his progress. Still another glanced over his shoulder as he ran from imaginary lions.

The team of neuroscientists was inspired to conduct their study after reviewing footage of a particular sleep study subject—an ex-smoker who dreamt of putting out a cigarette in an ashtray and mimed the motion with his hands and head while sleeping [see video; note: the glowing light near his fingers is not a cigarette, but an oxygen sensor]. What if, the researchers wondered, his eyes were moving in sync with his body to complete the action?

The new study by Arnulf and her colleagues suggests that's exactly what his eyes were doing. If rapid eye movements during REM sleep were truly random twitches they would not match their accompanying dream actions so frequently: For example, the ex-smoker might mime putting out a cigarette in an ashtray to his right, even as he looked to the left. Instead, it seems our eyes engage directly with the images in our dreams, just as our eyes engage with our environments when awake.

"Rapid eye movements are not random," Arnulf said. "If you are kissing someone in your dream, your eyes are directed towards the person you are kissing. The dream world is just fantastic."