Question

为什么不睡觉why not sleep?或是why don't go to sleep?表达都正确吗？

为什么还不睡觉，why not sleep?或是why don't go to sleep?
这两种方式表达都正确吗？
why not是什么用法呢？后加什么？why not 一般是表达建议的吧？
如果说why not go to sleep也对吧？

Answer 1

why not后面直接接动词原形， 但why don't后面要有主语的，所以题目中why don't go to sleep是错误的，只能说why don't you go to sleep或why not (go to) sleep?

Answer 2

why not do sth
why don't you do sth

Answer 3

why not 这种用法太生硬，不礼貌，推荐使用建议语气
I think it`s time to sleep,right?
I think U`d better to sleep.

Answer 4

why not sleep?比why don't go to sleep?更简单明了，多用于日常交流。所以第一种上正确的。