Pupillary dilation at a given luminance is a proxy for norepinephrine levels, with larger pupils indicating higher arousal.

“if you measure pupilary pupil dilation that's a pretty good proxy for NE for the neurogen neric system right so at a given people will wonder what um how to do it we're not going to um go into too much detail here but at a given brightness in the room what we call luminance um the peup pool tends to be smaller when it's bright and larger when it's um you're in a dim room that's sort of obvious um but at a given luminance the more alert aroused somebody is um arousal as a general term here then the pupil tends to be more dilated it gets bigger the more um norepinephrine is is in the system”