i sleep for eight hours.中的for必须用吗?

i sleep for eight hours.中的for必须用吗? Usually I sleep for eight or nine hours a night.中为什么要用介词for而 I think it's important for us ____(sleep) eight hours a nigh students n＿＿＿ sleep for eight hours We should sleep at ( ) for eight hours a day I sleep for nine hours every night sleep的用法I have sleeping for 9 hours.I have slept for 9 hours 分析I slept for eight hours last night 句子成分 it is n_ for you to sleep eight hours every day. I never sleep for less than nine hours.中文是什么? I sleep eight hours a day 就eight提问 sleep for 20 hours sleep 20 hours 哪一个更正确