In other words, sleep well! You often hear this phrase as part of the rhyme: ‘good night, sleep tight, don’t let the bedbugs bite.’
Here is one common theory about the source of the phrase. Early beds had ropes tied across the wooden frame in a criss-cross pattern. A straw mattress was then put on top of the ropes. Over time the ropes stretched, causing the bed to sag. The owner would then tighten the ropes to get a better night’s sleep.
However, according to another source, ‘sleep tight’ didn’t derive from ancient furniture and is, in fact, a comparatively modern expression. The earliest citation to be found is from 1866. In her diary Through Some Eventful Years, Susan Bradford Eppes included:
“All is ready and we leave as soon as breakfast is over. Goodbye little Diary. ‘Sleep tight and wake bright,’ for I will need you when I return.”
There aren’t many other known citations until the early 20th century and the OED lists none until 1933. Rope-strung beds were long out of date by this time, so it’s unlikely they gave rise to the ‘sleep tight’ phrase.
The word ‘tightly’ used to mean ‘soundly, properly, well.’ An earlier phrase, ‘tight asleep,’ derives from this meaning, as seen in this example from Marie Beauchamp’s novel Elizabeth and her German Garden, 1898:
“And once, when there was a storm in the night, she complained loudly, and wanted to know why lieber Gott didn’t do the scolding in the daytime, as she had been so tight asleep.”
The phrase ‘sleep tight’ itself was common in the 20th century, and was used in the lyrics of a song at the height of Beatlemania: Lennon and McCartney sang Good Night on the White Album in 1968:
Now it’s time to say good night,
Good night. Sleep tight.
I should have written this post late at night instead of in the morning; I’m making myself sleepy!