Unless you have specific contact lenses that are approved for overnight wear, it is NOT OKAY to sleep in your contact lenses!  When you sleep in them a few things happen: bacteria adhere to the lens surface and increase your risk of eye infection, your eyes dehydrate and the contact lens can cause small abrasions, and most commonly, your eye doesn't get enough oxygen (your closed eyelids when you are sleeping means that your cornea doesn't get as much oxygen as when your eyes are open).  When this happens, you can develop serious infections that can cause discomfort, light sensitivity, scarring and even blindness.

Related: What is the harm in extending the life of my contact lenses?