Donne didn’t invent the comparison between death and sleep, but he uses it here to great effect. But, you have to know a tiny bit of Christian theology to fully understand the idea. It is thought that, when faithful Christians die, they are only "dead" until the Day of Judgment comes and Christ returns to Earth. They compare this length of time to a period of "sleep." At this point, time ends, eternity begins, and all the faithful Christians who died will "wake up" to be led into Heaven. At this point, all their earthly troubles are over for good, and they will be at "rest" with God.
- Line 5: This metaphor compares "rest" and "sleep" to "pictures," like a painting or drawing. The point is that the rest and sleep are pale imitations, and Death is the real thing. On the other hand, Death is only a much stronger version of sleep, and not something scary and different.
- Line 8: This line describes what the experience of death means to the "best men" of line 7. One of its meanings is eternal rest for their weary bodies, or "bones."
- Line 11: The comparison between Death and sleep becomes an extended metaphor at this point. The speaker says that, if he only wants a really good sleep, he doesn’t even need Death; he can use "poppies" (opium, a kind of drug) or "charms" (magic or potions).
- Line 13: The extended metaphor continues. He calls the time between the speaker’s death and the Day of Judgment a "short sleep." In human terms, this may not seem that short (we can assume the speaker is "asleep" for hundreds of years already), but, compared to Eternity, pretty much anything is short. When the speaker "wakes up," he will find himself in Heaven.