Cats are naturally nocturnal, and are skilled hunters any time of day -- but at night their eyes kick into high gear. Their eyes are highly sensitive to light, and are equipped with pupils that can dilate dramatically. They also have a component called the tapetum, which offers double light exposure. These characteristics make cats' eyes appear large at night.
The Nature of Eyes
Your pupils dilate in the dark, and the same goes for cats. This is nature's way of adjusting the amount of light allowed into the eye. The pupil -- the black circle in the middle -- controls how much light is allowed into the eye. In bright light, pupils get smaller so the eyes are not overwhelmed with light. In the dark, they widen to let more light in. The muscles of the iris -- the colored part of the eye -- control pupil size.
Hunting in the Dark
Cats are carnivores, and as a result they are natural hunters. This is reflective in the size of their eyes. The most evident feature of nocturnal hunters is eye size, says BioMedia Associates, a website offering programs for biology education. Cats have wide pupils, large eye lenses and other features that let them collect information about the area surrounding them. At night, these features kick into action even more, widening their eyes to allow vision in low light.
Double Light Exposure, Plus Maximum Dilation
Cats' impressive night vision is due partially to a part of their eye called the tapetum. This reflective layer is under the retina, which contains cells called rods and cones; these absorb light and transform it into information the brain can process. When light reaches the tapetum, it reflects back to the rods and cones. Therefore, cats' eyes have double light exposure. This also gives cats their characteristic "shine" at night. Cats can also dilate their pupils significantly, maximizing the amount of light coming through at night.
Exciting Hunts, Big Nights, Restful Days
Cats' eyes can also dilate due to adrenaline, produced during "fight or flight" responses -- for example while he tracks down that mouse behind the heating vent. His pupils can also dilate if he's injured or on certain medicines. Just as his eyes widen to maximum size at night, they retract significantly during the day, when your cat's pupils often shrink to nothing more than a small vertical slit. Cats also lay low during the day to protect their eyes from too much light.