Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve. This response results in either constriction (miosis), narrowing the pupil, or dilation (mydriasis), widening the pupil. This occurs via relaxation or contraction of the iris dilator muscle respectively. 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 sexual stimulation. The pupils contract immediately before someone falls asleep. A pupillary response can be intentionally conditioned as a Pavlovian response to some stimuli.