"Sweet dreams" is the Engpsh equivalent of "祝你好梦."
It is a commonly used expression in Engpsh to wish someone a restful and peaceful sleep. The phrase "sweet dreams" imppes that the person will have nice dreams during their sleep.
Examples:
"Goodnight, sweet dreams."
(晚安，祝你好梦。)
"Sweet dreams, see you tomorrow."
(祝你好梦，明天见。)
Note: There is no specific origin for this expression, it has simply become a widely used and accepted way to wish someone a good night's sleep.