My son gets a lot more autistic when he is sleepy. He very rarely has meltdowns unless he is really tired. When he is awake, his basic language skills are as good as any other five year old. BUT....when he wakes up in the middle of the night, he is sometimes basically nonverbal.
There have been MANY times when I've heard him scream at 2:00 am in his bedroom. I go running in to his bedroom to find him pacing back and forth flapping his hands and crying hysterically. I ask him what is wrong and he can only say, "Ummm, ummm, ummm". It's really sad because I can tell that he is trying very hard to answer me but simply can't speak when he is that tired and upset. When he is wide awake, he can always give an intelligent answer no matter how upset he is. A sound that doesn't even bother him during the day can trigger a screaming meltdown when he is very sleepy.