What is the origin of 'sleep tight'?

Read moreOne of the mysteries of the English language finally explained.

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An equation in which one quantity is expressed in terms of a sum of other quantities; specifically any of several standard formulae which express a trigonometrical function of a sum or a difference of angles as a sum or difference of products of similar functions of the individual angles.

Late 19th century; earliest use found in Proceedings of the Royal Society of London.