The phrase "goodnight, sleep tight" is a common bedtime wish, but its origin is somewhat debated. One popular theory suggests that the phrase originates from the way beds were constructed in the past. In the 16th and 17th centuries, mattresses were often supported by a latticework of ropes or tapes that needed to be tightened regularly to provide a firm sleeping surface. 

"Sleeping tight" would mean sleeping on a well-tied, firm bed. The word "tight" in the 18th and 19th centuries also meant "securely" or "soundly." So "sleep tight" could simply mean "sleep securely" or "sleep soundly." 

This usage of "tight" can still be seen in expressions like "sit tight." It's also possible that the phrase "sleep tight" became popular simply because it rhymes and sounds pleasant. The combination of "goodnight" and "sleep tight" creates a rhythmic and memorable phrase, which helped it become a common bedtime wish.