A survey by the Centers for Disease Control and Prevention of 1,000 adults found 99 per cent were breaking one of the golden rules for wearing contacts.
The number of contact wearers has grown steadily over the last 20 years with 3.5 million people in the UK and 30 million in the U.S. choosing the soft lenses instead of glasses to correct their eyesight.
The report found a number of worrying - but extremely common practices - such as failing to clean them properly.
More than half of wearers admitted they had slept overnight wearing their contacts while almost nine in 10 people had taken a nap with their lenses in.
The bad habits - such as topping up rather than replacing contact lens solution - can lead to potentially devastating eye infections like keratitis - inflammation of the cornea which can cause blindness.
Nearly a third of wearers admitted they had gone to a doctor because of red or painful eyes, the report said.
However the study's biggest concern was the number of people who exposed their contact lenses to water - by either rinsing their lenses in tap water, showering or going swimming.More than eight in ten people admitted showering while wearing them and six in ten went swimming while still wearing contacts.
More than a third of people said they had rinsed their lenses under a tap while almost 17 per cent said they had soaked them in tap water overnight.
Experts said this significantly increased the risk for infection because it can transfer micro-organisms living in water to the eye.
Although household tap water is safe for drinking, it is not sterile and contains bugs that can contaminate contacts and cause eye infections.
The infection-causing bugs can be found in tap water and swimming pools, which are then transferred to the eye through a dirty lens.
Keratitis can also be caused a scratch from a fingernail, while inserting or taking out lenses.
Meanwhile, half of wearers confessed they had kept their current lenses past the recommended date instead of replacing them as advised.
The CDC made a number of recommendations to keep infection risks to a minimum, including:
Previous research in Australia also found good hygiene was vital to avoiding infections.
It discovered people were five times more likely to get an infection if they bought lenses on the internet as there were fewer reminders about how to keep them clean.
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