jBRaD's Things You Didn't Know: Why do we say, "sleep tight"?

Why do we say, "sleep tight"?

There are two explanations. The first, from "Uncommon Goods":

In the days before box springs, mattresses were supported by taught ropes, stretched between bed frame rails. When ropes loosened over time, mattresses lost support and sagged, making bedtime particularly uncomfortable. Tightly pulled ropes became synonymous with restful sleep, hence the nighttime saying "sleep tight!" .

Fun answer, eh? Unfortunately, history has a way of spoiling cool explanations. That old spoilsport, the Oxford English Dictionary, provides a much more prosaic explanation.

"It seems that tight in this expression is the equivalent of the only surviving use of the adverb tightly meaning 'soundly, properly, well, effectively'."

No ropes, no saggy mattresses, no bedbugs. Sorry 'bout that. But now you know.