It's an allusion to a Christian legend about seven Christian youths who hid in a cave in hopes of avoiding the persecution of Decius in AD 249 in Ephesus. Eventually, they were caught, tortured and were walled up alive in the cave they once hid. However, miraculously, they did not die, slept for 187 years and were awaken in AD 479, a new world where Christianity became the major religion.

Same imagery with the poem of waking up in a new world.

Source : https://catholicsaints.info/seven-sleepers-of-ephesus/