go to bed vs. go to sleep

go to bed 是就寢，go to sleep是入睡。我們可以確知
自己幾時、幾分、甚致幾秒就寢，但卻無法確知自己
幾時、幾分入睡。

例如：I always go to bed at 11 o’clock every night.
I don’t know when I went to sleep last night.