Scientists at the University of Geneva have performed a study in asleep mice that reveals pupil diameter changes during sleep. They say this mechanism protects the brain during deep sleep and that measuring pupil size during sleep could be a less invasive readout of brain activity.
Yüzgeç, Ö., Prsa, M., Zimmermann, R., & Huber, D. (2018). Pupil Size Coupling to Cortical States Protects the Stability of Deep Sleep via Parasympathetic Modulation. Current Biology. doi:10.1016/j.cub.2017.12.049