The answer to this question depends a lot on your own personal tear chemistry and eye characteristics, as well as the type of contacts you use. In other words, you will have to ask your own eye doctor
There are some contacts that are made to be left in while you sleep and there are other contacts that are definitely not made to be worn while asleep. If you have the former type of contact, as long as you properly clean then and dispose of them when it is time for a new pair, then it should be fine to sleep with them in. On the other hand, if your contacts are not made to be worn around the clock, then you could be potentially damaging your eyes by leaving them in.
Even in the case of contacts that are made to be worn continuously, there are some people who, because of their personal eye characteristics, cannot tolerated wearing them like this. In other words, use common sense - if your eyes feel irritated after sleeping with your contacts in, then this is probably not right for you.
Your eye doctor will be able to provide you with more individualized advice.