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It is difficult to know exactly what this "sleep" is an allegory to. Certainly it means that he has lost God's path. The quote goes,
"How I got into it I cannot say,
Because I was so heavy and full of sleep
When first I stumbled from the narrow way..."
The "narrow way" probably comes from Matthew (Bible),
"Enter through the narrow gate; for the gate is wide and the way is broad that leads to destruction...."
The sleep part of this means loss of direction on this path so the closest allegory seems to be the Bible.