Every night, Ralph's sleep patterns must have been getting worse due to a desire to escape.
"Ralph settled himself for his nightly game of supposing."
"Nightly" suggests frequency here, and "supposing" suggests purpose: to escape.
He begun again to dream about the ponies and the cottage where he and his folks would live. In this cottage lifestyle, he would find comfort, something far removed from his current life. He would experience the challenge of the ponies, riding and caring for them. He mentions Dartmoor and Devon, obvious locations that he has fond recollections of or at least he longs to go there.
This section is so significant because he is longing for anything that is the opposite of the current savage experience.
"His mind skated to a consideration of a tamed town where savagery could not set foot. What could be safer than the bus center with its lamps and wheels?"
This dream gives him the opportunity to escape for just moments, but the greater impact is the fact that Ralph can't escape physically at all. He is tied into this savagery whether he wants to be or not and the impact of evil is going to keep infiltrating the boys unless relief arrives. He allows his mind to travel because it is the only part of him that can, but that is not good because when his mind is "away" from his body, he cannot defend himself from the savagery around him.
He continues in and out of this dream as his body wakes him because it twitches and spasms while he dreams. This suggests either a vivid dreamer or a tortured sleeper or both. Ralph so longs for civilization that his dream cites the excitement of riding on a bus of all things.