The rhyming phrase good night, sleep tight is a remnant of an archaic sense of tight meaning soundly, properly, or well.
The use of tightly in this sense dates to Elizabethan times. From Shakespeare’s 1598 The Merry Wives of Windsor:
Hold Sirha, beare you these Letters tightly.
The use of tight, without the -ly, appears by 1790 when it is used in a poem by James Fisher:
I charg’d them tight, An’ gart them pay o’ lawing clink, Mair than was right.
The familiar rhyme appears by the late 19th century. From a poem published in the 9 August 1897 Naugatuck (Connecticut) Daily News:
As from the staircase she would call:
“Good night, sleep tight, my dear!”
I heard one false explanation for the origin of the term while visiting Shakespeare’s birthplace in Stratford-on-Avon. The tour guide (tour guides are notorious sources of apocryphal information) told us that the sleep tight derived from the fact that Elizabethan beds had a foundation consisting of a rope net. When the bed began to sag, one would tighten the net. Well, such beds do exist (there is one in the Bard’s birthplace), and the use of tightly to mean soundly does date to Elizabethan times, but as we have seen the rhyme and specific association with sleep appears much later.
Copyright 1997-2015, by David Wilton