I'm going to sleep, is what you tell others when you expect to fall asleep now. If you say it to yourself, with a strong emphasis on the word going (may vary with the individual), then you are mad because you can't sleep and you will do it no matter what. of course, you can't see this meaning from the written words. You will only know that from hearing the words and the inflections . I will go to sleep, is future tense and means that you expect to fall asleep in the future, preferably soon. Some will argue that the difference is like splitting hairs, but thought I'd let you know there are other possibilities.

They are okay sentences. They don't mean the same thing, but don't know if you intended for them to be the same.