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.

“These dynamics reflect the state of arousal, or the level of brain activation in regions that are responsible for sleep-wake regulation,” says Lustenberger. “These observations contradict the previous assumption that, essentially, the level of arousal during sleep is low.”

Instead, these fluctuations in pupil size indicate that even during sleep, the brain continuously shifts between higher and lower levels of activation. These findings also confirm in humans what other research groups have recently observed in rodent studies, where slow fluctuations in activation levels (known as arousal) were detected. The ETH researchers’ study was recently published in the journal Nature Communications.

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