Eye pupils change size even when animals are asleep – and it could be a defence mechanism

You can tell a lot from someone’s eyes. Beyond responding to changes in light, it’s been claimed pupil dilation can indicate a range of different psychological states from arousal to shifting attention and vigilance. It’s rather apt, then, that it’s this pair of organs that have often called the window to the soul.

Now, new research has shown that pupil size could also tell us what your brain is doing when you’re asleep, as well as when you’re awake. Well, if you’re a mouse, that is.

After discovering his laboratory mice sometimes slept with their eyes open, Assistant Professor Daniel Huber and his team at the University of Geneva in Switzerland decided to study the mice’s pupil size as they dozed.

To start, the team developed a system that involved placing an infrared light near the mouse’s head. When the eyes were viewed with an infrared camera, the pupils could be seen as bright white circles, enabling them to accurately measure any changes.

The team’s work paid off because rather than just changing at random, pupil size in the mice was found to reliably indicate the animals’ sleep state.

“We found pupil size rhythmically fluctuates during sleep,” Huber explains. “Intriguingly, these pupil fluctuations follow the sleep-related brain activity so closely that they can indicate with high accuracy the exact stage of sleep – the smaller the pupil, the deeper the sleep.”

After carrying out further experiments, the team was able to demonstrate that pupil size was controlled by the parasympathetic autonomic nervous system, the same part of the nervous system that controls activities like digestion and sexual arousal when the body’s at rest. The conclusion is that in mice, at least, pupils narrow in deep sleep to protect the animals from waking up with a sudden flash of light.

This seemed slightly puzzling to me and I asked Huber why mice would have evolved to not wake up in conditions that might often indicate danger.

“Since sleep in general, but in particular phases of deep NREM sleep, are thought to be crucial for memory consolidation, our results suggest that constricting the pupil during deep sleep has evolved to protect these important moments,” he explained.

“Also, periods of deep sleep always alternate with periods of lighter sleep where potential danger is more easily detected. However, there is always a trade-off when gating potentially relevant sensory stimuli.”

It’s a fascinating discovery and naturally, Huber and his team are keen to see if their findings also apply to humans. If this is the case, it’s possible the pupil tracking method could also be adapted for use in sleep clinics.

“We are currently discussing with sleep laboratories how to study similar phenomena in humans. Patients sleeping naturally with their eyelids open (nocturnal lagophthalmos) might give us first insights very soon,” he told Alphr.

“Inferring brain activity by non-invasive pupil tracking might be an interesting alternative or complement to electrode recordings.”

Image credit: Özge Yüzgeç, Mario Prsa, Robert Zimmermann, Daniel Huber