What to watch out for if I sleep with contact lens on?

Not a good idea. You are at 40 times greater risk of serious corneal ulcer. Patient can get corneal barasion, feels like sand in the eye. Corneal ulcer usually feels the same but with more pain.
Do not . Do not sleep with contacts in, but if you do then signs of a problem would be pain, redness, blurred vision. Anything out of the ordinary.

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What to watch out for if I sleep in contact lenses?

Infection. I do not recommend sleeping in contacts on a regular basis. I like to see patients think of sleeping in contacts as an option, but on most nights take them out. The risk of a painful, expensive and potentially vision threatening infection goes up significantly when you sleep in your contacts. And just because you have been doing it for years does not decrease your risk. Read more...