Oyasuminasai or Oyasumi?

[linguaholic]

Is there only a difference of formality when using Oyasuminasai or Oyasumi=? As far as I know they both stand for 'Good Night'. I am not an expert in Japanese but my educated guess would be that Oyasuminasai is the more formal version of Oyasumi? -->because of the suffix 'nasai'. 

What is/are the Kanjis for Oyasumi? @Blaveloper would you be so kind to introduce some sample sentences. That would be appreciated. 

 

[宇崎ちゃん]

Sample sentences for "good night" are tricky in any language.
But indeed, "oyasumi" is more commonly used to friends and family, while "oyasuminasai" is more commonly used to guests, teachers, bosses, etc.

There is a Kanji (お休み), but it's often used in Hiragana alone.

[wanderer.girl]

They both mean 'Good night' but differ in their level of courtesy and politeness. I was also confused at first of the difference between these two but learned that 'Oyasuminasai' is the more formal/polite usage and 'Oyasumi' is more casual. 

Yes, Oyasuminasai for bosses, teachers and the like.
Then just drop the 'nasai' and simply use 'Oyasumi'  if you are bidding goodnight to family or friends.
 

[linguaholic]

Dear everyone

I have a nice update on this one. 

If you are truly interested to find out about the real and full meaning about Oyasumi and Oyasuminasai, please have a look at this article on our blog: Sleep Tight: The Whole Meaning of Japanese "Oyasumi"

[reader]

I like the article posted. Thank you for that.

To a boss, I would not order, even politely, to do anything. A better way might be simply saying 明日も宜しくお願いします (ashita mo yoroshiku onegai shimasu).

お休みなさい is really more of a set phrase that I would not consider overly polite.