Who controls pupils size?

Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae.
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What does the size of your pupils mean?

As you'll have noticed, the same pupil response can mean different things, although generally dilated pupils send a positive message and when they constrict it's a negative one. But exactly what it means depends on the situation (and whether someone has turned on a light).
  • Do your pupils dilate when you drink alcohol?

    Slower pupil reaction–Alcohol causes the iris toconstrict and dilate at a much slower speed. This becomes an issue for drivers who have been drinking as they cannot adapt as quickly to oncoming headlights.
  • How do your pupils dilate?

    Muscles in the colored part of your eye, called the iris, control your pupil size. In low light, your pupils open up, or dilate, to let in more light. When it's bright, they get smaller, or constrict, to let in less light. Sometimes your pupils can dilate without any change in the light.
  • Do your pupils get bigger when you lie?

    So if you ask someone to tell you their favorite color and the year they were born, their response times should be about the same. Pupil dilation is a reliable indicator of lying since enlarged pupils are a sign that your brain is working hard—which it has to do in order for you to tell a lie.

Updated: 17th October 2019

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