What is the "sleepers' den" in Donne's poem, The Good-Morrow?

The "seven sleepers' den" (4) is a reference to a story taken from Christian martyrology, in which seven brothers were forced to hide in a cave to avoid being persecuted for their faith. By the grace of God and in respect for their faith, these children were able to sleep peacefully in their cave for more than 200 years before awakening safely. The sleepers' den, therefore, refers to a state of waiting in anticipation of eventually awakening to fulfill the true purpose of one's life: in this case, true love.