Cách sử dụng
The expression "sleep tight" is used as a way to say "sleep well" or "have a good night's sleep". It is typically used when saying goodbye to someone who is going to bed. The phrase dates back to the time when mattresses were supported by ropes that needed to be tightened to make the bed more comfortable to sleep on. So, "sleep tight" originally meant to sleep on a bed that was tightened properly. Nowadays, it is just a warm and friendly way of wishing someone a good night's sleep.