What does Ralph's sniggering about being rescued suggest about his state of mind?
I think this certainly suggests that Ralph is beginning to lose it. All sense of hope for rescue is gone, it is as if he has been overtaken by the beast, the Lord of the Flies, and/or evil.
To me, this also suggests a loss of control. In the beginning, the boys believed in Ralph to keep order. Their confidence helped Ralph believe in himself too, but after some time, civilized boys gave way to savagery. As they left him for Jack, Ralph slowly lost control and with it his focus and control over his own mind. Now, a course for destruction has been set and Ralph doesn't know how to turn it around.
Ralph's body is beginning to twitch, tremble, and spasm. These seem uncontrollable and they wake him from sleep. His mind's ability to remain sober and think straight has left him whether because of emotional or physical forces. This is similar to what Simon experienced near the Lord of the Flies.