Saying "Good Night" in Japanese

"Now we are going to learn how to say good night. Good night in Japanese is oyasumi-nasai. Repeat after me oyasumi-nasai. The same as good morning you can also use just oyasumi for casual good night. Its real meaning is have a good sleep, a good rest. So as in English you have to differentiate good evening and good night. If it is 9 pm if you are meeting a person for the first time that day, you can say good evening. But if you are saying good bye at 9 pm that would be oyasumi-nasai. So let's practice; just imagine it is 7 pm and you walk into a restaurant where I work how do you say? Yes that is right good evening. Now you are leaving the restaurant after a meal what do you say? Oyasumi-nasai it sounds really good."