Sleep tight: This phrase means to sleep well. The origin of the term is unclear, but one common theory suggests that it comes from the way beds were constructed in the past, with a mattress supported by ropes or cords. Tightening these ropes would provide a firmer sleeping surface. There is no solid proof for this explanation, and the first written mention of this phrase was published long after mattresses were made in this manner by Susan Bradford Eppes.
