“I accidentally slept in my contacts. Can just once cause permanent damage?”

Yes. Even one night sleeping in contact lenses can result in complications that can lead to permanent loss of vision, most commonly by corneal infection. There are many contact lenses that are FDA approved for overnight wear with an acceptable safety profile. But, no contact lens is completely free of risk and proper care of your lenses and quick evaluation if you are having a problem is critical in those rare cases when something goes wrong. Fortunately, most people do well, but don't risk your vision by waiting if you are having a problem. Call your eye care specialist.

Swelling should resolve. Be aware that sleeping over night with contacts dramatically increases risk for corneal infection and ulcers

Swelling is a sign of inflammation which can be due to toxins released by bacteria or a bacterial infection of the conjunctiva or the cornea. Also oxygen deprivation can be a cause. Consult your ophthalmologist.

Sometimes, yes, just once can cause permanent damage.
Every time you sleep in your contact lenses you run the risk of getting a serious infection, microabrasions, and a corneal ulcer with scarring -- or worse. The cornea does not have blood vessels in it and only gets oxygen and nutrients through the tear film. When you sleep in a contact lens it can suck on to the cornea and not allow exchange of nutrients and oxygen because the tear film doesn't get to the cornea. Even without a contact lens, the normal oxygen content drops from about 96% to 53% while you're sleeping, so imagine how much lower it is with the contact lens in place. Although people get away with it, it is sort of like the chances you take when you ride a motorcycle in a dangerous place without a helmet; you may get away with it sometimes but the one time you're in an accident it's often fatal.

If you wake up with your eyes swollen or red, that indicates that something is wrong, even if it's not painful. (Swelling around the cornea can numb the sensation). If there is any blurred vision, redness around the cornea, or pain, you should definitely seek professional help immediately. If there is just a little swelling then try using a lot of artificial tears and keep the contacts out and see if you get better in an hour or two. If not, you should definitely seek professional attention because the earlier and infection is caught the better the outcome.
In general, with contact lenses, if there is any irritation blurred vision excessive tearing or any other issue, it is best to follow the adage "if in doubt, take it out".

I hope this is helpful.

It may. Sleeping in contacts decreased oxygen getting to your corneas. This can make corneas swell and cause irritation that can make kids swell. Keep contacts out and see an eye care professional because you could also potentially have corneal ulcer which needs to be treated with antibiotics

Sleeping in contacts can lead to an overwear temporary change in the eyes usually due to oxygen deficiency. This can cause irritation, pain, redness and less commonly lid swelling.  These are all temporary and usually takes 1-2 days to disappear. No permanent changes result if this is all that happened. Rarely, an infection might occur - causing redness that is generalized, pain and sometimes lowered vision. Looking in the mirror you might notice a white dot on the cornea (the window at the front of the eye upon which the contact lens rests). If this is noted, see an ophthalmologist right away. Otherwise if the symptoms disappear in 2 days then, you can go back to your usual contact lens schedule. Of course, don't wear the lens until the eye clears.  

Usually not, but repetitively can cause damage, sometimes permanent damage to the cornea and tear producing structures.

Roger Ohanesian, MD

Not if it is a soft lens.

No, you may have a cornea ulcer. This is an emergency. You can lose your vision. You need to see an ophthalmologist ASAP. And please do not use the contact lens again until further instructions from the ophthalmologist.

Usually it will if you don't get an infection. Stay off contacts for a few days for eye recovery.