0404 GMT January 18, 2020
They are a thin, curved lens that is designed to sit on a film of tears covering the surface of the eye, express.co.uk reported.
In the UK there are approximately three million users.
There are a number of options for wearers, including lenses worn for one day or a month and hard or soft.
However, using them doesn't come without its risks and according to the National Health Service (NHS), many people don't use them safely.
Indeed, it was reported earlier this week that a 67-year-old woman had to have 27 contact lenses removed from her eye.
According to the British Medical Journal, they had stuck together to form a large mass which was spotted during routine cataract surgery.
While this was an extreme case, it's not unusual.
Brendan O'Brien, COO of Vision Direct, said, "It isn’t uncommon for a contact lens wearer to arrive at their optometrist’s practice early on in the morning having forgotten or been unable to remove a contact lens late on the previous evening.”
The NHS recommended that either extended wear contact lenses —which can be worn continuously for a period of time — or daily wear contact lenses should be worn overnight.
O'Brien warned, "Wearing lenses which are not designed for extended wear overnight can lead to infections, corneal ulcers and other health problems that can cause permanent vision loss.
"Contact lenses reduce the much-needed supply of oxygen to the cornea, or the surface of your eye.
"Normally, the cornea gets oxygen both from blood vessels in the eyelid at night and from the air during the day.
"A regular contact lens user relies on the night-time supply to keep the eye healthy, so cutting off night-time oxygen can be devastating."
Indeed, according to the British Contact Lens Association the already high risk of infection from wearing them during the day is much higher if they're slept in.
If you don't know whether you've left a lens in or not, possibly because it's slipped off the center of the eye, you can look for certain signs.
O'Brien explained, "The first and most obvious is discomfort.
"The other is that if you wake up and can see without putting on your glasses, you should make sure you haven’t forgotten to remove your lenses the previous evening."