There's an average pupil size that's considered normal. Measured in millimeters, your eyes' pupils can change based on light, medications, and even emotions.

The maximum pupil size also varies significantly among age groups. For example, the pupil is the widest around age 15, after which it begins narrowing in an inconsistent fashion after age 25.

When the orexin neurons in the brain are stimulated, pupils expand, new research in mice shows. Orexin is central to the regulation of pupil size. When the orexin system is switched off, the ...

The way the brain regulates pupil size is different from previously thought: fundamentally responsible is the neurotransmitter orexin, as researchers at ETH Zurich have now shown.