Pupillary Response

“Pupillary response is a physiological response that varies the size of the pupil, either resulting in constriction or dilation. The response can have a variety of causes, from an involuntary reflex reaction to exposure or inexposure to light — in low light conditions a dilated pupil lets more light into the eye — or it may indicate interest in the subject of attention or indicate sexual stimulation. The pupils contract immediately before someone falls asleep.?