Wearing eye contacts when yoou are sleeping will lead to the insufficiency of oxygen in the eyes as well as eye dehydration, which can cause many eye diseases such as conjunctivitis, keratitis, dry eyes, ophthalmic nerves paralysis, and so on. But it is rare to cause astigmatism by wearing eye contacts when you are sleeping. Astigmatism refers to the refractive error that the lights through the flextion of different warps in the eyes can not locate in the same focus on the retina, which makes the image on the retina become blurred. Some people are born to have astigmatism, while some people get the astigmatism due to overuse of the eyes, or unhealthy reading habits.
Yes, it is really bad for your eyes when you sleep with contact lenses in the eyes. However it will not be possible for you to get astigmatism which is usually born to be with. However, your eyes will get infection because of wearing contact lenses in the eyes. You should keep hygiene when you wear the contact lenses. You need to protect the eyes carefully.
There are several brands of extended wear contact lenses for astigmatism that are approved by the FDA for overnight wear, including Acuvue Oasys for Astigmatism (Vistakon), Hydrasoft Toric XW (CooperVision) and PureVision Toric (Bausch Lomb). There also are rigid gas permeable contact lenses for overnight wear that correct astigmatism.