My answer will be from the US colloquial language use. If someone is going to sleep, a friend might say to another friend, "Get a good night's sleep." Or "Get a good night's rest." Or the shorter phrase, "Sleep well." It's a warm response that wishes well to the other person, especially if their health is poor or they've had restless sleeping patterns. For the difference between "Have a good night" and "Good night", my answer is the same as Bob's.

It depends what you are going say, if someone is parting from you we would just say "Good night". If someone is going out we would say "Have a good night." Hope this helps Bob