Sleeping in your contact lenses is bad for your eye health. The risk increases the longer you sleep in the lenses. 10 to 15 minutes might not put your eyes in too much danger, although your eyes will feel a little sore. A couple of hours is enough to cause serious damage to your eyes.
If you've accidentally fallen asleep in your contact lenses, try not to remove them immediately as they'll be too dry. Apply some eye drops and blink several times to reintroduce moisture to the lenses. Once your eyes are more hydrated, gently remove the contacts. If you can, don't put a fresh pair of lenses in immediately, allow your eyes time to recover first.