10 June 2021
What is the science behind cats’ eyes lighting up?
If you’ve ever been out in the dark, you might have noticed a pair of eyes lit up in the shadows, staring at you intently. It can be a little unsettling, until you realise it’s just the neighbour’s cat watching you from the bushes. Thousands of years of ago, Ancient Egyptians and Ancient Greeks believed, respectively, that cats could capture sunlight during a sunset and hold it in their eyes until morning, and that our feline friends had their own light sources within their eyes. As lovely an explanation as this is, we now actually know the science behind the phenomenon of glowing eyes!
Cats’ eyes glow in the dark because they reflect light. This is because they are nocturnal animals, meaning they hunt in the dark, and the phenomenon of their reflective eyes (called tapetum lucidum) boosts their night vision significantly. This is why other nocturnal animals also have eyes which appear to glow in the dark. There is a reflective structure at the back of a cat’s – or other nocturnal animal’s – eye which acts as a mirror, bouncing light off of it and reflecting it back out of the animal’s eye. The tapetum lucidum also absorbs any light which escapes the retina, giving cats vision almost 50% better than they would otherwise have. The light reflecting out of the cat’s eye is what we see when their eyes appear to be glowing in the dark. Their eyes actually act as two little torches, guiding the way when they’re hunting in the dark.
Why don’t humans’ eyes glow in the dark?
Unlike cats, humans don’t have a reflective layer at the lack of the eye. This is because we are naturally active in daytime, making the most of daylight, and therefore don’t rely on tapetum lucidum to improve our night vision.
Our pupils involuntarily dilate and contract to allow different levels of light into our eyes, for example contracting in bright settings to allow less light in and protect our eyes from damage. However, cats can actually use the muscles in their eyes to control the level of light which their eyes are subjected to. If you’ve ever seen a cat with slit-shaped pupils in their eyes, it is so they absorb less light in that moment.
Humans have generally poor night vision but, while cats’ night vision is much better than ours, they still struggle to distinguish shapes and sharp lines when it’s dark, resulting in blurry vision. Nevertheless, they make use of twice as much light as humans and only need one-sixth of the illumination level that humans need.
What do different coloured glowing eyes mean?
The glow that we see from a cat’s eye can appear as different colours and you would usually see them as blue, green or yellow. Cats’ eyes are made up of molecules, riboflavin and zinc, and the levels of zinc play a part in determining which colour their eyes appear. Similarly to new born babies, most of whom are born with blue eyes which later darken, kittens are born with blue eyes. Because of this, younger cats will always have glowing eyes which appear blue, although the colour is likely to change as they grow older.
Younger cat’s eyes are also likely to glow brighter than those of older cats due to the tapetum lucidum becoming less powerful over the years. This can mean that their eyes appear dimmer as the years go by, but it isn’t something to worry about. If your cat’s eyes appear red, especially in the dark, it could also suggest that light is no longer reaching the tapetum lucidum, something which is particularly common with older cats.
According to Senior Cat Wellness, “Red eyes could also be a result of a surprising light source. If a cat’s eyes were not expecting to be flooded with light, they will rapidly dilate, leading to a bloodshot appearance. This should be a temporary and not a regular aesthetic.”
What if my cat’s eyes glow in the daytime?
Simply put, your cat’s eyes should never glow in the daytime. The glow would indicate that their pupils are dilated and are allowing high levels of light into the eye, which is something that shouldn’t happen during daylight settings. This could suggest a problem with their eye(s), so it’s important that they get checked by a vet if this occurs.
Conversely, your cat’s eyes should always appear reflective when they are in dark settings. If your cat’s eyes don’t glow when they’re in dim lighting, they likely have a problem with their vision. This could be glaucoma, cataracts or conjunctivitis and, again, it’s important that they are seen by a vet to determine and treat the problem.