Pupil Size May Indicate Stages of Sleep

Our pupils are always changing size to regulate how much light is let into our eyes when we’re awake. Walk outside into the bright sun and they shrink down to a dot. Enter a dark movie theater and they open up again. They even change along with our alertness or attention levels. But what happens to our pupils when we’re asleep? Do they still change their size, and if so, why?

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A team from the University of Geneva led by NYSCF — Robertson Investigator Daniel Huber, PhD, found that changes in the size of a mouse’s pupils during sleep correlate to what stage of sleep the mouse is in. The research, published in Current Biology, described the finding that as the mouse fell deeper into sleep, its pupils got smaller. The researchers think this could be a way for the parasympathetic nervous system to protect the mouse from waking up from deep sleep to a jarring amount of light. And the same could be happening in humans.

The challenge in studying pupil size during sleep lies in finding a reliable method of tracking the pupils while the eyelid is down. The researchers found a way to do this by shining an invisible infrared light through the head of the mouse to illuminate the back of the eye. Then, they observed the eye through an infrared camera, which registered the pupils as bright circles. Using this system as the mouse slept, the researchers could measure the mouse’s pupil size as it progressed through the different sleep stages.

Building on this work, the researchers next plan to test whether the same patterns are seen in humans. If so, then measuring pupil size using an infrared light could be a useful method for determining sleep state and could serve as an alternative to electrode recordings.

For more information on this study, check out the paper in Current Biology, or read more on EurekAlert