Pupil size indicates brain activity in sleep
Your pupil size might indicate the sleep state

Daniel Huber and colleagues from the University of Geneva recently discovered that the pupil size fluctuates rhythmically with sleep. They suggest that the pupil size is a consistent indicator of sleep stage and the brain activity is correlated with the pupil size during the non-rapid eye movement (NREM) sleep. These fluctuations seem to be regulated by the parasympathetic system of the brain. This has interesting implications since one might be able to deduce brain activity non-invasively by pupil tracking which can be used as alternatives to electrode recordings.

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Scientific publication: www.cell.com