As convenient as contact lenses are, it is very important to remember that they are a FDA approved medical device. Therefore, it is important to take proper care and handle your contact lenses safely.
If you overwear your contact lenses (ie. sleeping in them when advised not to) or clean and handle them improperly, it could lead to red eyes, infections or even ulcers. If any of those things become severe, they can even lead to blindness!
Therefore, whenever you are using your contact lenses, make sure you are throwing them out at the proper time. For example, even if you open a monthly lens and only use it three times, it is still important to throw them out at the end of the month. Also ensure that you are cleaning them with your doctor's prescribed cleaning solution and making sure that they soak in the solution for the proper amount of hours.
If you have any questions make sure to contact your eye doctor!