YOU SAID:

I could sleep for 12 hours.

INTO JAPANESE

１ ２時間も眠れなかった

BACK INTO ENGLISH

I couldn't sleep for 12 hours.

INTO JAPANESE

１ ２時間も眠れなかった。

BACK INTO ENGLISH

I couldn't sleep for twelve hours.

INTO JAPANESE

私は１ ２時間も眠れなかった。

BACK INTO ENGLISH

I couldn't sleep for twelve hours.