It is time for bed. 是睡觉的时候了。
I’ve put the children to bed. 我已安置孩子们睡觉了。
He sat by the bed, watching TV. 他坐在床边看电视。
比较 go to bed(上床睡觉)与 go to the bed(到床边去)。如：
He went to bed at 10 last night. 他昨晚10点钟上床睡觉。
He went to the bed and found a red box on it. 他走到床边，发现床上有个红盒子。
有时用不用冠词与搭配有关，比较：
It’s a bad habit to read in bed. /It’s a bad habit to read
on a [the] bed. 躺在床上看书是个坏习惯。
注：in bed 通常不用冠词，on the [a] bed 通常要用冠词。