ETH Zurich researchers found that pupil size changes during sleep, indicating shifts in brain activity. This could help diagnose sleep and neurological disorders.

Our eyes are typically closed when we sleep. However, beneath our closed eyelids, a flurry of activity takes place. A team of researchers, led by principal investigators Caroline Lustenberger, Sarah Meissner, and Nicole Wenderoth from the Neural Control of Movement Lab at ETH Zurich, has observed that pupil size fluctuates constantly during sleep. Sometimes it increases, sometimes it decreases—sometimes these changes occur within seconds, while other times they unfold over several minutes.