Good night, when translated to Nihongo, it becomes 'Oyasumi nasai' or 'Oyasumi'. The difference between "oyasumi nasai" and "oyasumi" is that 'oyasumi nasai' is a more polite way in saying good night.
In Hiragana (a kind of Japanese writing or calligraphy), it is written as "おやすみなさい(Oyasumi nasai)" or "おやすみ(Oyasumi)"
In Kanji (a more complicated Japanese writing), it is written as "お休みなさい(Oyasumi nasai)" and "お休み(Oyasumi)"
Note their differences.
In Hiragana (a kind of Japanese writing or calligraphy), it is written as "おやすみなさい(Oyasumi nasai)" or "おやすみ(Oyasumi)"
In Kanji (a more complicated Japanese writing), it is written as "お休みなさい(Oyasumi nasai)" and "お休み(Oyasumi)"
Note their differences.