Help me to understand: おやすみ vs. おやすみなさい
 

おやすみ versus おやすみなさい.

Which one would I use, is おやすみなさい more formal? What is the difference?

~Thanks.

おやすみなさい is just more formal yes.

Both of them are used as "sleep tight" or "good night" or something to that effect. Literally, the former means rest (noun) while the latter is an imperative.

Please correct me if I'm wrong. :p

Correct.

@OP: General rule of thumb for Japanese greetings: The longer it is, the more formal it tends to be. 'Polite' greetings are also usually longer.

Then again, this is true for greetings in most languages as well.

「おやすみ」 also has other meanings, such as "Holiday" or "Vacation".

I think 「おやすみ」, when used as a greeting, should be avoided when speaking with strangers (or people you do not know well.) I may be wrong in this case though, so it's best to check with a native speaker.

Umm but for the imperative form I do not think you would attach お at the beginning. Thus, "Go to bed!" or "Take a break!".　would be　休みなさい！ not おやすみなさい。

The imperative mood, despite the name, does not necessarily have to refer to commands, and may refer equally to requests as well.

Attaching 「-てください」, 「-いなさい」 and 「-てちょうだい」 (among others) changes the mood of a phrase into the imperative mood. Since this is a 'grammatical concept', (and if I recall correctly), this remains true regardless of the speaker's intention.

Now, most Japanese would neither interpret 「おやすみなさい」 as a command nor as a request, but then again the interpretation of a lot of common phrases also differ from their literal meanings (「ありがとう」、「こんにちは」、
「すみません」、「ごめんなさい」, for example.). Furthermore, Robhol used the word "literally".

As such, I believe that robhol is technically correct.

TL;DR version: 「おやすみなさい」=Good night. It literally means "Please rest." 「おやすみ」 also means "Good night", but is less formal than 「おやすみなさい」.

I was just saying they do not attach an お at the beginning of imperative phrases... have you ever heard of someone saying おたべなさい？ In the same way I do not think おやすみなさい is ever used to have an imperative meaning. It is just politely saying "good night".

http://www.google.com/search?hl=en&c...9D&btnG=Search

Yes, it is a polite way of saying "good night", and while it is neither an order nor a request, it is still, technically, in the "imperative mood".

Of course, I'm splitting hairs at this point, and this argument is no doubt detrimental to the OP's understanding (and hence should be ignored by the him) but I would like to reiterate that robhol was technically correct.

お待ちください...ever seen this phrase? Give a look at the different forms of 敬語 (けいご) and you will answer your question on your own.
There are ways to use "imperative" also when you have to be polite, they just have a different "color".
Now, some of those are used into regular talk (the way I see it) just like お休み(ください not there, but might as well be).
EDIT: Let me get better what I meant. Honorific and humble verbs are made adding the お to the pre ます form and なさる is the honorific form of the verb to do. Basically what you do (I hope I am not wrong) is transform the verb into a noun and then add the verb "to do". Not much different from those many verbs made of a Chinese origin kanji noun+する like 勉強する.
As per お食べください, the verb to eat in humble and honorific changes completely, that's why the example you made wouldn't work.
I hope I am not too far off :)

I am not sure what we are arguing about. My statement simply was that you cannot construct an imperative form （なさい） and put お at the beginning of the verb and sound natural. So if you wanted to use 休む in the imperative form （なさい） you would write it 休みなさい　not　おやすみなさい。just like you would not say お行きなさい　お飲みなさい、お運びなさい。I mean, i can confirm this with japanese friends to be sure, and if I am wrong i am wrong, but my feeling is that polite お and imperative -なさい do not mix.