The expression “sleep tight” is used as an affectionate way of wishing someone a good night’s sleep. To sleep peacefully and without waking. It is frequently used as an alternative to saying good night.
“sleep tight”这个短语用来表示“（睡得）很香，很甜，不被打扰”。它通常用来代替晚安。
Examples:
例如：
I will see you in the morning. Sleep tight.
我早上会看你，晚安。
Get some rest, we have a busy morning tomorrow. Sleep tight.
休息一下，睡个好觉。我们明早会很忙。
Good night, sleep tight. I hope you have wonderful dreams.
晚安，睡个好觉，祝你能做个好梦。
This expression originated back to when mattresses were supported by ropes instead of planks of wood or bars on your bed, “sleeping tight” would refer to sleeping with the ropes pulled tight which would provide a well sprung bed. The phrase is also part of a very well know children’s rhyme, “good night, sleep tight, don’t let the bedbugs bite”. This rhyme, and the phrase “sleep tight”, are commonly used as an alternative way of saying “good night” in nearly all English speaking countries.
这个表达可以追溯到以前海军用绳子来代替床上的木板或木条支撑床垫的时候，“sleep tight”指的是“睡在拉紧的绳索上”，它相当于一个很好的弹簧床。这个短语还被用在一首著名的儿歌里“睡个好觉，别让臭虫咬”。这首儿歌以及“sleep tight”这个短语在很多国家中相当于“good night”。
There are many idiomatic expression in the English language, some of which are used in certain settings. Most have emerged from particular events, they are useful to learn if you want to get a greater understanding of the English language.
英语中有很多地道的表达，其中有一些适用于特定的场景。大多数表达都来源于特定的事件，如果你想对英语有更深入的了解，学习这些表达是很有用的。