| This line is taken from a tune called Goree Blues, attributed (according to at least one source) to inmates at the Goree State Farm near Huntsville, Texas. The full couplet is:
Lay down last night with nothing on my mind.
Woke up this morning, blues all 'round my bed.
It's also classic pentameter, as white bread as cucumber sandwiches and as old as the language itself. Compare to these famous lines from Hamlet:
For in that sleep of death what dreams may come,
When we have shuffled off this mortal coil,
The question is: Why? It's tempting to imagine a few educated slaves or sharecroppers spreading this meter throughout black southern culture, but there's just no evidence for it.
It may be that pentameter is the most "natural", or conversational, rhythm of English. You hear it a lot: in slogans, popular songs, in business writing, and in day-to-day speech. In this theory, the delta blues grew out of the natural music of casual spoken English.
This theory has a parallel in the idea of convergent evolution, where similar structures (like fins) show up on different species because they're the best ways of dealing with similar environmental pressures. Pentameter may be the best way of dealing with the problem of "how do I make English sound good?" | 
