English or Japanese that you will never learned from the book.

Today, I was asked this morning how are you and I said "I'm sleepy" then one of the students told me 早くねったら。。。
I was stunned for almost 10 seconds.
then I understood what are they trying to say. after that Teacher K asked me how to say it in English. 早くねったら　is a conditional form means "if you sleep early" however if you say it to foreigner maybe we will wait what are the next sentence you will say because it incomplete.
It is either you say "If you sleep early your not sleepy now."
or
"then sleep early."

Everytime I talked with others, I learned new things that you may not seen in the book.