Every so often, my 4 year old son will suddenly begin laughing and giggling hysterically while lying in bed. He often has just been woken up, or has woken up, and will fix his eyes on something off into space, and will just laugh like crazy. He is awake, and answering our questions, but continues to laugh. It is really creepy. Communication is difficult with him because his speech is delayed. Any idea on why this happens? It has never happened before with our other children.